lmdb: clean testdb-dir while "make test".
lmdb: more usefull info from mdb_stat tool.
lmdb: get a lag of the reading for dreamcatcher.
slapd: dreamcatcher feature.
lmdb: oomkiller feature.
slapd: oomkiller feature.
back-mdb: checkpoint-interval in seconds instead of minutes.
back-mdb: support for "checkpoint kbytes" config-option.
lmdb: properly sync meta-pages in mdb_sync_env().

Meta-pages may be updated during data-syncing in mdb_sync_env(),
in this case database would be inconsistent.

Check-and-retry if lead txn-id changed during flushing data in mdb_sync_env().
lmdb: MDB_LIFORECLAIM & MDB_COALESCE modes.

Reclaim FreeDB in LIFO order - this is a main feature.
Also aim to coalesce small FreeDFB records.
lmdb-backend: support config for 'lifo' and 'coalesce' envflags.
lmdb: lock meta-pages in writemap-mode to avoid unexpected write,
before the data pages would be synchronized.

Without locking the meta-pages may be writen by OS before other data,
in this case database would be inconsistent.
back-mdb: stack backtrace on SIGSEGV and friends (optional).