s3: Open gencache_notrans with TDB_NOSYNC

s3: Open gencache_notrans with TDB_NOSYNC

We are doing CLEAR_IF_FIRST here, and we are doing the transactions only to
protect gencache_stabilize against concurrent writers. tdb's transaction.c
says:

  - if TDB_NOSYNC is passed to flags in tdb_open then transactions are
    still available, but no fsync/msync calls are made.  This means we
    are still proof against a process dying during transaction commit,
    but not against machine reboot.

For gencache_notrans.tdb this is exactly what we want and avoids some expensive
disk syncs.

Reviewed by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Nov 28 03:13:56 CET 2012 on sn-devel-104