LP#1482757: More careful Located URI remapping

LP#1482757: More careful Located URI remapping

Instead of recreating all Located URI mappings, we will add any new
ones and remember all those (existing and new) that are in use. Any
existing maps that we don't see in this record, and whose URI is only
used once previously and therefore about to be orphaned, we remove.

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>