This PR is related to #3107 and a continuation of #3351

It is important to emphasise that in the privval original design, client/server and listening/dialing roles are inverted and do not follow a conventional interaction.

Given two hosts A and B:

    Host A is listener/client
    Host B is dialer/server (contains the secret key)
    When A requires a signature, it needs to wait for B to dial in before it can issue a request.
    A only accepts a single connection and any failure leads to dropping the connection and waiting for B to reconnect.

The original rationale behind this design was based on security.

    Host B only allows outbound connections to a list of whitelisted hosts.
    It is not possible to reach B unless B dials in. There are no listening/open ports in B.

This PR results in the following changes:

    Refactors ping/heartbeat to avoid previously existing race conditions.
    Separates transport (dialer/listener) from signing (client/server) concerns to simplify workflow.
    Unifies and abstracts away the differences between unix and tcp sockets.
    A single signer endpoint implementation unifies connection handling code (read/write/close/connection obj)
    The signer request handler (server side) is customizable to increase testability.
    Updates and extends unit tests

A high level overview of the classes is as follows:

Transport (endpoints): The following classes take care of establishing a connection

    SignerDialerEndpoint
    SignerListeningEndpoint
    SignerEndpoint groups common functionality (read/write/timeouts/etc.)

Signing (client/server): The following classes take care of exchanging request/responses

    SignerClient
    SignerServer

This PR also closes #3601

Commits:

* refactoring - work in progress

* reworking unit tests

* Encapsulating and fixing unit tests

* Improve tests

* Clean up

* Fix/improve unit tests

* clean up tests

* Improving service endpoint

* fixing unit test

* fix linter issues

* avoid invalid cache values (improve later?)

* complete implementation

* wip

* improved connection loop

* Improve reconnections + fixing unit tests

* addressing comments

* small formatting changes

* clean up

* Update node/node.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_client.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_client_test.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* check during initialization

* dropping connecting when writing fails

* removing break

* use t.log instead

* unifying and using cmn.GetFreePort()

* review fixes

* reordering and unifying drop connection

* closing instead of signalling

* refactored service loop

* removed superfluous brackets

* GetPubKey can return errors

* Revert "GetPubKey can return errors"

This reverts commit 68c06f19b4.

* adding entry to changelog

* Update CHANGELOG_PENDING.md

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_client.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_dialer_endpoint.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_dialer_endpoint.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_dialer_endpoint.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_dialer_endpoint.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* Update privval/signer_listener_endpoint_test.go

Co-Authored-By: jleni <juan.leni@zondax.ch>

* updating node.go

* review fixes

* fixes linter

* fixing unit test

* small fixes in comments

* addressing review comments

* addressing review comments 2

* reverting suggestion

* Update privval/signer_client_test.go

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

* Update privval/signer_client_test.go

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

* Update privval/signer_listener_endpoint_test.go

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

* do not expose brokenSignerDialerEndpoint

* clean up logging

* unifying methods

shorten test time
signer also drops

* reenabling pings

* improving testability + unit test

* fixing go fmt + unit test

* remove unused code

* Addressing review comments

* simplifying connection workflow

* fix linter/go import issue

* using base service quit

* updating comment

* Simplifying design + adjusting names

* fixing linter issues

* refactoring test harness + fixes

* Addressing review comments

* cleaning up

* adding additional error check

In order to re-enable the test harness for the KMS (see
tendermint/kms#227), we need some marginally more realistic proposals
and votes. This is because the KMS does some additional sanity checks
now to ensure the height and round are increasing over time.

This issue is related to #3107
This is a first renaming/refactoring step before reworking and removing heartbeats.
As discussed with @Liamsi , we preferred to go for a couple of independent and separate PRs to simplify review work.

The changes:

    Help to clarify the relation between the validator and remote signer endpoints
    Differentiate between timeouts and deadlines
    Prepare to encapsulate networking related code behind RemoteSigner in the next PR

My intention is to separate and encapsulate the "network related" code from the actual signer.

SignerRemote ---(uses/contains)--> SignerValidatorEndpoint <--(connects to)--> SignerServiceEndpoint ---> SignerService (future.. not here yet but would like to decouple too)

All reconnection/heartbeat/whatever code goes in the endpoints. Signer[Remote/Service] do not need to know about that.

I agree Endpoint may not be the perfect name. I tried to find something "Go-ish" enough. It is a common name in go-kit, kubernetes, etc.

Right now:
SignerValidatorEndpoint:

    handles the listener
    contains SignerRemote
    Implements the PrivValidator interface
    connects and sets a connection object in a contained SignerRemote
    delegates PrivValidator some calls to SignerRemote which in turn uses the conn object that was set externally

SignerRemote:

    Implements the PrivValidator interface
    read/writes from a connection object directly
    handles heartbeats

SignerServiceEndpoint:

    Does most things in a single place
    delegates to a PrivValidator IIRC.

* cleanup

* Refactoring step 1

* Refactoring step 2

* move messages to another file

* mark for future work / next steps

* mark deprecated classes in docs

* Fix linter problems

* additional linter fixes

* WIP: Starts adding remote signer test harness

This commit adds a new command to Tendermint to allow for us to build a
standalone binary to test remote signers such as KMS
(https://github.com/tendermint/kms).

Right now, all it does is test that the local public key matches the
public key reported by the client, and fails at the point where it
attempts to get the client to sign a proposal.

* Fixes typo

* Fixes proposal validation test

This commit fixes the proposal validation test as per #3149. It also
moves the test harness into its own internal package to isolate its
exports from the `privval` package.

* Adds vote signing validation

* Applying recommendations from #3149

* Adds function descriptions for test harness

* Adds ability to ask remote signer to shut down

Prior to this commit, the remote signer needs to manually be shut down,
which is not ideal for automated testing. This commit allows us to send
a poison pill message to the KMS to let it shut down gracefully once
testing is done (whether the tests pass or fail).

* Adds tests for remote signer test harness

This commit makes some minor modifications to a few files to allow for
testing of the remote signer test harness. Two tests are added here:
checking for a fully successful (the ideal) case, and for the case where
the maximum number of retries has been reached when attempting to accept
incoming connections from the remote signer.

* Condenses serialization of proposals and votes using existing Tendermint functions

* Removes now-unnecessary amino import and codec

* Adds error message for vote signing failure

* Adds key extraction command for integration test

Took the code from here:
https://gist.github.com/Liamsi/a80993f24bff574bbfdbbfa9efa84bc7 to
create a simple utility command to extract a key from a local Tendermint
validator for use in KMS integration testing.

* Makes path expansion success non-compulsory

* Fixes segfault on SIGTERM

We need an additional variable to keep track of whether we're
successfully connected, otherwise hitting Ctrl+Break during execution
causes a segmentation fault. This now allows for a clean shutdown.

* Consolidates shutdown checks

* Adds comments indicating codes for easy lookup

* Adds Docker build for remote signer harness

Updates the `DOCKER/build.sh` and `DOCKER/push.sh` files to allow one to
override the image name and Dockerfile using environment variables.
Updates the primary `Makefile` as well as the `DOCKER/Makefile` to allow
for building the `remote_val_harness` Docker image.

* Adds build_remote_val_harness_docker_image to .PHONY

* Removes remote signer poison pill messaging functionality

* Reduces fluff code in command line parsing

As per
https://github.com/tendermint/tendermint/pull/3149#pullrequestreview-196171788,
this reduces the amount of fluff code in the PR down to the bare
minimum.

* Fixes ordering of error check and info log

* Moves remove_val_harness cmd into tools folder

It seems to make sense to rather keep the remote signer test harness in
its own tool folder (now rather named `tm-signer-harness` to keep with
the tool naming convention). It is actually a separate tool, not meant
to be one of the core binaries, but supplementary and supportive.

* Updates documentation for tm-signer-harness

* Refactors flag parsing to be more compact and less redundant

* Adds version sub-command help

* Removes extraneous flags parsing

* Adds CHANGELOG_PENDING entry for tm-signer-harness

* Improves test coverage

Adds a few extra parameters to the `MockPV` type to fake broken vote and
proposal signing. Also adds some more tests for the test harness so as
to increase coverage for failed cases.

* Fixes formatting for CHANGELOG_PENDING.md

* Fix formatting for documentation config

* Point users towards official Tendermint docs for tools documentation

* Point users towards official Tendermint docs for tm-signer-harness

* Remove extraneous constant

* Rename TestHarness.sc to TestHarness.spv for naming consistency

* Refactor to remove redundant goroutine

* Refactor conditional to cleaner switch statement and better error handling for listener protocol

* Remove extraneous goroutine

* Add note about installing tmkms via Cargo

* Fix typo in naming of output signing key

* Add note about where to find chain ID

* Replace /home/user with ~/ for brevity

* Fixes "signer.key" typo

* Minor edits for clarification for tm-signer-harness bulid/setup process