FEATURE - lmdb: MDB_LIFORECLAIM & MDB_COALESCE modes.

 Reclaim FreeDB in LIFO order - this is a main feature.
 Also aim to coalesce small FreeDFB records.
 FEATURE - lmdb-backend: support config for 'lifo' and 'coalesce' envflags.
 BUG FIX - lmdb: 'volatile' to important fields, which
           may be updated by readers asynchronously.

 Without 'volatile' compiler may eliminate a mdb_find_oldest() calls.
 BUG FIX - lmdb: reordering of instructions which update the txn in a meta-page.

 Without "volatile" or memory-barrier compiler may reorder instructions
 for update the "mm_txnid" field in meta-page in "writemap" mode.

 From the reader's point of view this cause a short
 time interval when the transaction is corrupted.
 BUG FIX - 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.
 EXTENSION - lmdb-backend: stack backtrace on SIGSEGV and friends (optional).