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

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