writemap: lock meta-pages to avoid unexpected write,
           before the data pages would be synchronized.
 lmdb-backend: stack backtrace on SIGSEGV and friends;
 ! debugging crutchs, MUST be removed before release.

   conditionally enforce MDB_LIFORECLAIM & MDB_COALESCE;
   test pattern hack (this crunch MUST be remove later);
   asserts for check reclaimable;
   inject SIGSERV on 42th checkpoint;