lmdb: implementation of "checkpoint kbytes",
       force flush when volume of the changes reached a configurable threshold.
 lmdb: more usefull info from mdb_stat tool;
 lmdb: properly sync meta-pages in mdb_sync_env(),

       check-and-retry if lead txn-id changed
       during flushing data in mdb_sync_env().
 lmdb-backend: support for "checkpoint kbytes" config-option.
 lmdb-backend: conditionally support config for 'lifo' and 'coalesce' envflags;
 ! conditionally enforce MDB_LIFORECLAIM & MDB_COALESCE (this MUST be remove from the release);
 ! test pattern hack (this crunch MUST be remove later);
 lmdb: MDB_LIFORECLAIM & MDB_COALESCE modes;