Table features

I believe this closes FlowForwarding/LINC-Switch#146. With this change, we delete flow entries in tables that are requested to be deleted. We still don't "delete" flow tables in any meaningful sense: since our implementation is not subject to hardware limitations, we treat all 255 possible flow tables as always in existence.

We could return only the requested tables in the table features response. I chose to return all tables including the supposedly deleted ones, as that shows the actual state of the switch. That could be changed if it causes problems.