replmd: Refactor logic to check if replicated link is newer

replmd: Refactor logic to check if replicated link is newer

This is precursor work for supporting single-link conflicts.

Split out the code to check if the link update is newer. It's now safe
to call this from the main codepath. This also means we can combine the 2
calls to get the seqnum into a single common call.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13055

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