lmdb: Simpler mdb_txn_commit().
lmdb: Simpler mdb_txn_commit().

mt_env is always set.
Commit(mt_child) resets mt_child, so parent need not.

Change-Id: Ib8af816a964a4b70fa6fa622b3d9dd463e9460b3

leo-yuriev/libfpta,leo-yuriev/libfpta