lmdb: lifo-2, assert for reclaimable;
 lmdb: lifo-3, reclaim free-db records from reader's tail.
 lmdb: aim to coalesce reclaimable records in freeDB.
 fix: 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.
 ! only test-060 (this crunch MUST be remove later);