Rework linked_attributes module for the REPLACE case.

This moves to a smarter 'find the delta' based operation of the linked
attributes module, when the caller asks for a 'replace' of the link
source.  Previously we would spray operations all over the database,
even if the net result was just to modify one record.

This also means we need the transaction safety less, which may be
useful for some LDAP backends that don't provide this functionality on
the LDAP server.

Andrew Bartlett
(This used to be commit 8c88e4eb1c0a606e7899091525260e8d6558ffd0)