Use a separate tdb for mutexes

Use a separate tdb for mutexes

Another preparation to convert secrets.c to dbwrap: The dbwrap API does not
provide a sane tdb_lock_with_timeout abstraction. In the clustered case the DC
mutex is needed per-node anyway, so it is perfectly fine to use a local mutex
only.
(This used to be commit f94a63cd8f94490780ad9331da229c0bcb2ca5d6)