add state sync reactor (#4705)

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.

rpc/core/status.go

@@ -31,7 +31,7 @@ func Status(ctx *rpctypes.Context) (*ctypes.ResultStatus, error) {
 	}
 
 	var latestHeight int64
-	if consensusReactor.FastSync() {
+	if consensusReactor.WaitSync() {
 		latestHeight = blockStore.Height()
 	} else {
 		latestHeight = consensusState.GetLastHeight()
@@ -66,7 +66,7 @@ func Status(ctx *rpctypes.Context) (*ctypes.ResultStatus, error) {
 			EarliestAppHash:     earliestAppHash,
 			EarliestBlockHeight: earliestBlockHeight,
 			EarliestBlockTime:   time.Unix(0, earliestBlockTimeNano),
-			CatchingUp:          consensusReactor.FastSync(),
+			CatchingUp:          consensusReactor.WaitSync(),
 		},
 		ValidatorInfo: ctypes.ValidatorInfo{
 			Address:     pubKey.Address(),