s3: Make connections_forall open connections.tdb r/w

s3: Make connections_forall open connections.tdb r/w

connections_forall is called from count_current_connections() which potentially
deletes dead records. This needs r/w access to connections.tdb.
connections_traverse says it does not provide this. Does not really matter in
the smbd case, because we have opened it before r/w, so this is "just" cleanup.