OVSDriver: factor out packet-in processing

This is going to get more complicated, so better to have it in its own file.
OVSDriver: use separate netlink sockets for packet-ins

Previously packet-ins were received by an upcall thread and queued to the main
thread. This extra queue was unncessary. Instead, we can create separate
netlink sockets for packet-ins and read them directly in the main thread. We
may also increase the number of packet-in sockets per port in the future, which
will allow better quality of service guarantees.
OVSDriver: replace upcall assert with AIM_ASSERT
Merge into master from pull request #186:
Remove extra level of queueing for packet-ins (https://github.com/floodlight/ivs/pull/186)