SwitchImpl: Update for new IOWorker

SwitchImpl had been broken by a recent refactor of
IOWorker.  This gets it working again.
datapaths: Add basic port_mod support to switch

Murphy: I did some surgery here to merge Peter's port_mod handling
for PORT_DOWN with the debugger branch's handling for NO_FLOOD.
I haven't tested it, but it should at least almost support both.