ITS#7968: add locking into syncrepl.

Mutex must be held even for a read-access of a cookie state,
as there may be a race with write in other thread.
mdb-backend: logging
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 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.
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: refine lifo freelist_save().
lmdb: care of mt_lifo_reclaimed for shared me_txn0.
syncrepl: ANY-logging for delete-ops.
syncrepl: ANY-logging for REFRESH.
 ! debugging crutchs, MUST be removed before release.

   LY_DEBUG;
   conditionally enforce MDB_LIFORECLAIM & MDB_COALESCE;
   test pattern hack (this crunch MUST be remove later);
   asserts for check reclaimable;
   inject SIGSERV on 42th checkpoint;
   added Yota's lmdb-tests;