ClosNetwork: move code to delDestinationsForPort()

This will be demanded by later commits.
ClosNetwork: select only uplink ports that are up
ClosNetwork: introduce port down event handler

Add new portWentDown() abstract method and overload it for spine and
leaf cases to handle only the ports where flushing destinations makes
sense.
ClosNetwork: handle uplink port down event

Reassign hosts that were assigned to the port to remaining uplink ports.
ClosNetwork: fix a bug in routePacket()

Always return an array -- that's what the calling method expects.
ClosNetwork: handle port up event

In particular, on 1st uplink port up try using it for all the
destinations that might be accumulated on the switch thus far.