lmdb: ITS#7974 get a lag of the reading for dreamcatcher.
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: checkpoint-interval in seconds instead of minutes.
lmdb: auto-sync with kbytes threshhold (aka checkpoint by kbytes).
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().
slapd: ITS#7974 dreamcatcher feature.
lmdb: ITS#7974 oomkiller feature.
slapd: ITS#7974 oomkiller feature.