namedb: increase lmdb mapsize limit to 100 MB
namedb: make the API constant
zone timers: just one resetting of timers
namedb: API change, allow init to return an error
timers: API change, allow returning of error code
timers: replace calculating memset with a function
timers: assertions to check API availability when db is non-null
LMDB namedb: error mapping from MDB_ error codes to KNOT_DATABASE_ERROR
timers: warn about error when opening the database
events: write persistent events from events API
timers: move database from config to server
timers: use new API in zone loading
zonedb-load: fix typo
tests-extra: add sleep before flush (case_sensitivity test)
review: replace KNOT_ERROR with lmdb_error_to_knot()
Merge branch 'persistent-timers-fixes' into '1.6'

fixes: persistent timers

See merge request !294