Interface still reflects cached assignments even after assigned Network object is deleted

[jathanism]

When a Network address is assigned to an Interface and the underlying Network object is deleted from the database, the Interface._addresses_cache and Interface._networks_cache are no updated. This is an inconsistency that needs to be addressed.

Possible solutions:

• A Network object cannot be deleted as long as it has an existing Assignment relationship to an Interface
• Deleting a Network with an Assignment in place also triggers an update to the Interface to refresh its cached relationships.