load existing zulip data into RChain as a snapshot (incrementally?)

[dckc]

The design as of Jun 17 (36f3190) only works if you start the mirror when you initially boot zulip.

But suppose you have an existing zulip DB? How about a way to load it into RChain as a snapshot?

suggested by @steverosstalbot

[dckc]

Perhaps the snapshot tool could work incrementally; i.e.

1. compare on-chain tables with off-chain tables; drop extras and create missing tables.
2. For each table, compare on-chain keys with off-chain keys; drop extras and add missing records.
3. For each record, compare on-chain data with off-chain data; replace any mismatched records.

Is there a way to avoid locking both sides for the duration? That could interfere with availability.

Perhaps timestamps could be used to optimize the record level data sync.

This could address the issue of mirror reliability (#49).