lmdb: lock meta-pages in writemap-mode to avoid unordered write.
lmdb: lock meta-pages in writemap-mode to avoid unordered write.

Without locking the meta-pages may be writen by OS before data pages,
in this case database would be inconsistent.

Change-Id: I6a262709b0741d7b57cb68231c4ae1c62a10a743

leo-yuriev/libfpta,leo-yuriev/libfpta