s4:dsdb:mod: repl_md: make message_sort transitive

s4:dsdb:mod: repl_md: make message_sort transitive

Before we had (with a TODO of regret):

       if (!a1 || !a2) {
               return strcasecmp(e1->name, e2->name);
       }

so, given {name:"A", id 2}, {name:"B", NO id}, {name:"C", id 1},

 A < B by name
 B < C by name
 A > C by id

Now the sort order is always A > C > B.

This sort could have caused mysterious crashes in repl_meta_data if
the schema is out of sync.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15625

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>