API for querying latest msg timestamps

[staltz]

We need a better API for querying the latest timestamps (or measuring "recency") of messages by author. We should not need to use internal ssb-db2 methods to achieve it.

I could do sbot.db.getLatest(authorId, (err, {offset,sequence} => but from the offset and sequence I can't get the message itself (in order to get the timestamp).

I'm currently doing it in a hacky way that uses internal APIs:

https://github.com/ssb-ngi-pointer/ssb-suggest-lite/blob/dbcc02ca35c134aceac45c855cd129dc695db8b7/src/index.ts#L129-L132