`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
Fixes#828. Adds state sync, as outlined in [ADR-053](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-053-state-sync-prototype.md). See related PRs in Cosmos SDK (https://github.com/cosmos/cosmos-sdk/pull/5803) and Gaia (https://github.com/cosmos/gaia/pull/327).
This is split out of the previous PR #4645, and branched off of the ABCI interface in #4704.
* Adds a new P2P reactor which exchanges snapshots with peers, and bootstraps an empty local node from remote snapshots when requested.
* Adds a new configuration section `[statesync]` that enables state sync and configures the light client. Also enables `statesync:info` logging by default.
* Integrates state sync into node startup. Does not support the v2 blockchain reactor, since it needs some reorganization to defer startup.
