contrib/lmdb: 0.9.18 -> 0.9.21

Maybe some coverity issues will get fixed, even though
they had seemed not to be affecting our use cases in kresd.

Tests+Deckard look OK and I've been using my system and kresd with
system-wide 0.9.21 for some time already.
5 jobs for lmdb-update
in 27 seconds and was queued for 2 seconds