EXTENSION - lmdb-backend: stack backtrace on SIGSEGV and friends (optional).
 ! 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;
backend += MDB_OOMKILL
lmdb += MDB_OOMKILL