Handle port_delete OF events
This is quite complicated... we need to handle port-flap events, or worse things where a VM reboots and then gets plugged into a different ofport. What do we call a flap, vs a time to revoke all caps owned by that port, or to that port? Stuff like that. Do we keep a shadow copy of switch, port, and node objects that is detached from the mul (OF) objects when they temporarily disappear, and then "reconnect" them when they reappear? We also need to add liveness information to these objects so that if people perform cap ops on a node cap they hold to a node that actually has gone away, we can deal with it. Same with flows...