Ref: #2563

I added IPC as an unencrypted alternative to SocketPV.

Besides I fixed the following aspects of SocketPV:

Added locking since we are operating on a single socket
The connection deadline is extended every time a successful packet exchange happens; otherwise the connection would always die permanently x seconds after the connection was established.
Added a ping/heartbeat mechanism to keep the connection alive; native TCP keepalives do not work in this use-case


* Extend the SecureConn socket to extend its deadline
* Add locking & ping/heartbeat packets to SocketPV
* Implement IPC PV and abstract socket signing
* Refactored IPC and SocketPV
* Implement @melekes comments
* Fixes to rebase

* WIP: switching to fixed offsets for SignBytes

* add version field to sign bytes and update order

* more comments on test-cases and add a tc with a chainID

* remove amino:"write_empty" tag

- it doesn't affect if default fixed size fields ((u)int64) are
written or not
- add comment about int->int64 casting

* update CHANGELOG_PENDING

* update documentation

* add back link to issue #1622 in documentation

* remove JSON tags and add (failing test-case)

* fix failing test

* update test-vectors due to added `Type` field

* change Type field from string to byte and add new type alias

- SignedMsgType replaces VoteTypePrevote, VoteTypePrecommit and adds new
ProposalType to separate votes from proposal when signed

- update test-vectors

* fix remains from rebasing

* use SignMessageType instead of byte everywhere

* fixes from review

* switch to amino for SignBytes and add Reply with error message

- currently only Vote is done

* switch Reply type in socket for other messages

 - add error description on error

* add TODOs regarding error handling

* address comments from peer review session (thx @xla)

 - contains all changes besides the test-coverage / error'ing branches

* increase test coverage:

 - add tests for each newly introduced error'ing code path

* return error if received wrong response

* add test for wrong response branches (ErrUnexpectedResponse)

* update CHANGELOG_PENDING and related documentation (spec)

* fix typo: s/CanonicallockID/CanonicalBlockID

* fixes from review

Currently the top level directory contains basically all of the code
for the crypto package. This PR moves the crypto code into submodules
in a similar manner to what `golang/x/crypto` does. This improves code
organization.

Ref discussion: https://github.com/tendermint/tendermint/pull/1966

Closes #1956

This reverts commit 58d0c8de89.

This reverts commit d02c5d1e30.

Also, fix consensus liveness issue.

This reverts commit 96a3502

This is a maintenance change to move the private validator package out
of the types and to a top-level location. There is no good reason to
keep it under the types and it will more clearly coommunicate where
additions related to the privval belong. It leaves the interface and the
mock in types for now as it would introduce circular dependency between
privval and types, this should be resolved eventually.

* mv priv_validator to privval pkg
* use consistent `privval` as import

Follow-up to #1255

Follow-up to feedback from #1286, this change simplifies the connection
handling in the SocketClient and makes the communication via TCP more
robust. It introduces the tcpTimeoutListener to encapsulate accept and
i/o timeout handling as well as connection keep-alive, this type could
likely be upgraded to handle more fine-grained tuning of the tcp stack
(linger, nodelay, etc.) according to the properties we desire. The same
methods should be applied to the RemoteSigner which will be overhauled
when the priv_val_server is fleshed out.

* require private key
* simplify connect logic
* break out conn upgrades to tcpTimeoutListener
* extend test coverage and simplify component setup

Follow-up to #1255 aligning with the expectation that the external
signing process connects to the node. The SocketClient will block on
start until one connection has been established, support for multiple
signers connected simultaneously is a planned future extension.

* SocketClient accepts connection
* PrivValSocketServer renamed to RemoteSigner
* extend tests

Following ADDR 008 the node will connect to an external
process to handle signing requests. Operation of the external process is
left to the user.

* introduce alias for PrivValidator interface on socket client
* integrate socket client in node
* structure tests
* remove unnecessary flag

As calls to the private validator can involve side-effects like network
communication it is desirable for all methods returning an error to not
break the control flow of the caller.

* adjust PrivValidator interface