accounting: time-based balance snapshots

Nobody has claimed this yet.

During the recent call with several `lnd` users, the notion of being able to obtain a balance "snapshot" at any period of time came up. I think the use case here is going back and looking at the historical flow of funds within a channel.

FWIW, this is already available in `lnd` as is, since we store the entire "revocation" chain. However this isn't indexed by "time" and instead uses an auto-incrementing sequence number of each new channel state. The only notion of time for the revocation chain is the absolute expiation heights on any HTLCs possibly present on the commitment transaction.

Alternatively, this can be done by polling `lnd` periodically, or having `faraday` use the streaming balance RPCs to update its own local state.

Start by reviewing how lnd stores the revocation chain and how faraday could consume the streaming balance RPCs. Clarify whether snapshots should be indexed by time or maintained as local state, and define the historical balance and fund-flow behavior that would count as complete.

Written by the indexing model from the issue text.