replmd: Only modify the object if it actually changed

replmd: Only modify the object if it actually changed

Commit 775054afbe1512 reworked replmd_process_link_attribute() so that
we batch together DB operations for the same source object. However, it
was possible that the object had not actually changed at all, e.g.
- link was already processed by critical-objects-only during join, or
- we were doing a full-sync and processing info that was already
  up-to-date in our DB.

In these cases we modified the object anyway, even though nothing had
changed. This patch fixes it up, so we check that the object has
actually changed before modifying the DB.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>