replmd: Skip redundant source object link checks

replmd: Skip redundant source object link checks

We receive the links grouped together by source object. We can save
ourselves some work by not looking up the source object for every single
link (if it's still the same object we're dealing with).

We've already made this change to replmd_process_linked_attribute().
This patch makes the same change to replmd_store_linked_attributes().
(We verify that we know about each link source/target as we receive each
replication chunk. replmd_process_linked_attribute() kicks in later as
the transaction completes).

Note some care is needed to hold onto the tmp_ctx/src_msg across
multiple passes of the for loop.

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