Add support for filtering PACKET IN messages

When we are facing overload situation with only a TCP connection, we cannot realistically sustain PacketIn messages, nor can we throttle the connection itself.

We end up throwing those messages away at OFP level, which means that they have gone through quite a bit of processing. It would greatly improve performance if we had a knob to OFJ, which would filter incoming requests as soon as they are identified as PacketIns, so they never make it out of the pipeline.