lmdb: clean testdb-dir while "make test".
 lmdb-backend: checkpoint-interval seconds instead of minutes.
 lmdb: implementation of "checkpoint kbytes",
       force flush when volume of the changes reached a configurable threshold.
 lmdb-backend: implement "checkpoint kbytes" option.
 lmdb: lifo-1, list of reclaimed freedb records.
 lmdb: lifo-2, reclaim free-db records from reader's tail.
 lmdb: aim to colease reclaimable records in freedb.