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.
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;