tdb: always open internal databases with incompatible hash.

This makes them more efficient due to better distribution
of keys across hash chains.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat Feb 15 08:26:07 CET 2014 on sn-devel-104