r13283: added two optimisations to the tdb transactions code. The first is to

r13283: added two optimisations to the tdb transactions code. The first is to
more agressively coalesce entries in the linked list of the undo
log. The second is to ensure that writes during a transaction into the
hash table don't cause the size of the undo log linked list to grow.

These optimisations don't affect Samba much, but they make a huge
difference to the use of ldb in kde