LP#1482757: Be sure to remove all orphaned URI

LP#1482757: Be sure to remove all orphaned URI

As reported by Michele Morgan, URIs can be orphaned in some cases, such
as when a location drops one of several 856 entries.  This commit tracks
objects at the mapping level, to identify disused URIs even where the
call number level is not changed.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Michele Morgan <mmorgan@noblenet.org>
Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Michele Morgan <mmorgan@noblenet.org>