replmd: rework replmd_modify_la_add to merge efficiently

replmd: rework replmd_modify_la_add to merge efficiently

Because both the list of added links and the list of existing links
are sorted, it is possible to interlace the two and obtain a merged
sorted list.

We avoid a great amount of talloc_realloc()ing by observing that the
merged list can't be longer than the sum of the two lists.

In the (common) case where there are many existing links but few being
added, we avoid parsing most of the existing link DNs and GUIDs if the
sorted_links feature flag is set.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>