VoxelPacketProcessor -> OctreePacketProcessor

added nodes bookkeeping in ReceivedPacketProcessor; added check in
sendNack() to not send NACKs to nodes that have sent packets that are
waiting in the message queue.
added default constructor for NetworkPacket

updated param comments in ReceivedPacketProcessor
moved sendNack() into packetDistributor()

...for the purpose of enforcing packets sent per interval in
OctreeSendThread.  Corrected mistake in keeping track of number of
special packets sent where sendSpecialPacket() was assumed to only send
one packet per call.
changed NetworkPacket::_sendingNode to ::_node
Merge remote-tracking branch 'upstream/master'
added killNode() slot for _packetCounts garbage collection
Merge pull request #3010 from wangyix/master

NACK packets are not sent to nodes with unprocessed packets; re-sent packets will be counted towards PPS