lmdb: more usefull info from mdb_stat tool.
lmdb: ITS#7974 get a lag of the reading for dreamcatcher.
lmdb: lock meta-pages in writemap-mode to avoid unordered write.

Without locking the meta-pages may be writen by OS before data pages,
in this case database would be inconsistent.
back-mdb: checkpoint-interval in seconds instead of minutes.
lmdb: auto-sync with kbytes threshold (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().
ITS#8011: few corrections for lmdb-backend;

 dn2id.c:
   + assertion for mdb_id2l_insert() result;
   - unnecessary assignment;

 search.c:
   - unused variable 'first';
   + range-check for mdb_idl_search() result;
ITS#5452, ITS#8012: fix syncops list iteration and detaching on freed;

Detaching a syncops record from op-list conditionally,
only when it was freed by syncprov_free_syncop.

The syncprov_drop_psearch() and syncprov_drop_psearch() now
returns a flag, which is nonzero if the given syncops was freed.
slapd: ITS#7974 dreamcatcher feature (refined).
lmdb: ITS#7974 oomkick feature.
slapd: ITS#7974 oomkiller feature.
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.
back-mdb: stack backtrace on SIGSEGV and friends.
back-mdb: stack backtrace enabled only if symbols available.
syncrepl: ANY-logging for delete-ops.
syncrepl: ANY-logging for REFRESH.