lmdb: bigbang for sync-to-disk path and related.
lmdb: bigbang for sync-to-disk path and related.

This is 4/9 for https://github.com/ReOpen/ReOpenLDAP/issues/1
and https://github.com/ReOpen/ReOpenLDAP/issues/2

Here a lot of changes:
 - deleted a secondary DSYNC-mode file descriptor;
 - deleted a pointers to meta-pages and toggle-selection of ones;
 - removed MDB_FDATASYNC_WORKS/FDATASYNC_MAYBE_BROKEN stuff;
 - reworked use of fdatasync/fsync for safety without performance degradation;
 - txn-to-meta updating moved info mdb_txn_commit();
 - removed mdb_env_write_meta();
 - rewrited mdb_env_sync0() for clarify;

Change-Id: I985464baa3cf486a2ceccf2c9dcb7a7fea698c46

leo-yuriev/libfpta,leo-yuriev/libfpta