figure out what to do about stale readers

[mykmelez]

LMDB > Caveats notes:

A broken lockfile can cause sync issues. Stale reader transactions left behind by an aborted program cause further writes to grow the database quickly…

Fix: Check for stale readers periodically, using the mdb_reader_check function or the mdb_stat tool.

We should figure out what to do about stale readers: whether to clear them out periodically ourselves or make this the responsibility of the consumer (exposing an API for them to do so).