dsdb: Rework kcc_deleted() into dsdb_garbage_collect_tombstones()

This is so that in a future commit, we can wrap this in python and allow it to be called
from outside the samba server processs.

This requires that we rework the callers and internals to avoid reference to
private data structures of the KCC service.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
