Snapshots of recent heights

[mappum]

To provide atomic queries, it will probably be useful to keep snapshots of recent states (say, for all blocks from the last minute). Then we can resolve queries against a given height as long as it is in snapshot range.

Since RocksDB snapshots are lost on restart, we will either have to not guarantee recent states are snapshotted (e.g. when resetting) or replay blocks to build them. I would lean toward the former since any new queries will be against the most recent block which will always have a snapshot (there is only a window of a few seconds after starting a node when there are no snapshots, clients can just retry).