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 from lmdb-update in 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
canceled #72934
docker linux amd64
build:linux:amd64

passed #20643
docker linux amd64
build:linux:amd64

00:00:24

 
  Test
passed #20645
docker linux amd64
deckard:linux:amd64

00:00:26

canceled #20646
docker linux amd64 allowed to fail
respdiff:linux:amd64

00:00:07

passed #20644
docker linux amd64
test:linux:amd64

00:00:26