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.
Tendermint RPC
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