can we (optionally) obtain full info from RocksDB indexes when using manysearch?

[ctb]

In the whole confusing mix of do-we-or-don't-we-need-sketches (links in #415 and documentation updates in #416), I'm guessing that one reason why multifastgather needs access to the full set of sketches (beyond the index) is because that way it can retrieve abundances.

Is it possible for us to update manysearch so that it can (optionally) retrieve abundances for sketches upon user request, if sketches are available in the RocksDB database? That makes manysearch much more useful...