Add filter to state sync

[ethanfrey]

There may be the desire to only sync a subset of data - like bank balances, or state of one contract.

Since we can sync current state and all updates on the fly easily, one can imagine that people would desire small custom programs to react to certain events. We could add an optional filter to the current state filter to only retrieve data under that prefix. And the updates to only return matching data (and maybe the _last_block metadata to track the height).

This could be a nice improvement in the future.