replmd: Group together link attribute processing by source object

replmd: Group together link attribute processing by source object

Instead of processing each link attribute one at a time, we want to
group them together by source object. This will mean we only have to
look-up the source object once, and only perform one DB 'modify'
operation. With groups with 1000s of members, this will help improve
performance.

This patch takes the first step of group together the links by
source-object. A new 'la_group' struct is added to help track what links
belong to the same source object. The la_list essentially becomes a
'list of lists' now.

Note that only related links *in the same chunk* are only grouped together.
While it is trivial to groups together links that span different
replication chunks, this would be a fairly insignificant efficiency gain,
but seems to have a fairly detrimental memory overhead, once you get
into groups with 10,000+ members.

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