Add API to query links regardless of validity

Links connect two persistent objects together via their UUID. This connection persists even when the RHS is not in memory. A consequence of this design choice is the possibility for "stale" links to persist, since there is no automatic link cleanup. This MR adds methods to query a link's RHS information, regardless of the validity of the link. This allows code that removes objects to clean up links even after the object has been removed.