ldb_kv_index: dn_list load sub transaction can re-use keys

ldb_kv_index: dn_list load sub transaction can re-use keys

We don't want to modify the original list, but we can reuse the keys
if we treat them as immutable and don't free them. That makes it a lot
quicker if there are many keys (i.e. where an index is useful) and may
sub-transactions. In particular, it avoids O(n²) talloc_memdups.

A removed comment that says "We have to free the top level index
memory otherwise we would leak", and this will be addressed in the
next commit.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15590

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>