## Description 

- Reduce duplication in messages and metrics. 
- merge WAL interfaces. Meant to push the developer to make changes in both places.

This cleans up the `Router` code and adds a bunch of tests. These sorts of systems are a real pain to test, since they have a bunch of asynchronous goroutines living their own lives, so the test coverage is decent but not fantastic. Luckily we've been able to move all of the complex peer management and transport logic outside of the router, as synchronous components that are much easier to test, so the core router logic is fairly small and simple.

This also provides some initial test tooling in `p2p/p2ptest` that automatically sets up in-memory networks and channels for use in integration tests. It also includes channel-oriented test asserters in `p2p/p2ptest/require.go`, but these have primarily been written for router testing and should probably be adapted or extended for reactor testing.

This revises the new P2P `Transport` interface and does some preliminary code cleanups and simplifications.

The major change here is to add `Connection.Handshake()` for performing node handshakes (once the stream transport API is implemented, this can be done entirely independent of the transport).  This moves most of the handshaking logic into the `Router`, such as prevention of head-of-line blocking, validation of peer's `NodeInfo`, controlling timeouts, and so on. This significantly simplifies transports, completely removes the need for internal goroutines, and shares common logic across all transports. This also allows varying the handshake `NodeInfo` across peers, e.g. to vary `ListenAddr`. Similarly, connection filtering is also moved into the switch/router so that it can be shared between transports.

This improves the `peerStore` prototype by e.g.:

* Using a database with Protobuf for persistence, but also keeping full peer set in memory for performance.
* Simplifying the API, by taking/returning struct copies for safety, and removing errors for in-memory operations.
* Caching the ranked peer set, as a temporary solution until a better data structure is implemented.
* Adding `PeerManagerOptions.MaxPeers` and pruning the peer store (based on rank) when it's full.
* Rewriting `PeerAddress` to be independent of `url.URL`, normalizing it and tightening semantics.

The `NodeInfo` interface does not appear to serve any purpose at all, so I removed it and renamed the `DefaultNodeInfo` struct to `NodeInfo` (including the Protobuf representations). Let me know if this is actually needed for anything.

Only the Protobuf rename is listed in the changelog, since we do not officially support API stability of the `p2p` package (according to `README.md`). The on-wire protocol remains compatible.

This implements a new `Transport` interface and related types for the P2P refactor in #5670. Previously, `conn.MConnection` was very tightly coupled to the `Peer` implementation -- in order to allow alternative non-multiplexed transports (e.g. QUIC), MConnection has now been moved below the `Transport` interface, as `MConnTransport`, and decoupled from the peer. Since the `p2p` package is not covered by our Go API stability, this is not considered a breaking change, and not listed in the changelog.

The initial approach was to implement the new interface in its final form (which also involved possible protocol changes, see https://github.com/tendermint/spec/pull/227). However, it turned out that this would require a large amount of changes to existing P2P code because of the previous tight coupling between `Peer` and `MConnection` and the reliance on subtleties in the MConnection behavior. Instead, I have broadened the `Transport` interface to expose much of the existing MConnection interface, preserved much of the existing MConnection logic and behavior in the transport implementation, and tried to make as few changes to the rest of the P2P stack as possible. We will instead reduce this interface gradually as we refactor other parts of the P2P stack.

The low-level transport code and protocol (e.g. MConnection, SecretConnection and so on) has not been significantly changed, and refactoring this is not a priority until we come up with a plan for QUIC adoption, as we may end up discarding the MConnection code entirely.

There are no tests of the new `MConnTransport`, as this code is likely to evolve as we proceed with the P2P refactor, but tests should be added before a final release. The E2E tests are sufficient for basic validation in the meanwhile.

closes: #5770
closes: #5769

also, include node ID in the output (#5769) and modify NodeKey to use
value semantics (it makes perfect sense for NodeKey to not be a
pointer).

`abci.Client`:

- Sync and Async methods now accept a context for cancellation
    * grpc client uses context to cancel both Sync and Async requests
    * local client ignores context parameter
    * socket client uses context to cancel Sync requests and to drop Async requests before sending them if context was cancelled prior to that

- Async methods return an error
    * socket client returns an error immediately if queue is full for Async requests
    * local client always returns nil error
    * grpc client returns an error if context was cancelled before we got response or the receiving queue had a space for response (do not confuse with the sending queue from the socket client)

- specify clients semantics in [doc.go](https://raw.githubusercontent.com/tendermint/tendermint/27112fffa62276bc016d56741f686f0f77931748/abci/client/doc.go)

`mempool.TxInfo`

- add optional `Context` to `TxInfo`, which can be used to cancel `CheckTx` request

Closes #5190