We want to be able to sync the current state without having to replay all transactions in the blockchain.

This can be done securely by first syncing a light client to a recent state root, and then polling peers for the pieces of the state tree.

Let's add a new state-sync reactor to Tendermint to handle this. It should use a new ABCI connection and message to ask the app what to ask other peers for - alternatively, there may be a case for it to just use the existing Query connection and message. The base of this could be thought of as a general purpose mechanism for letting apps control the fetching of data from Tendermint peers