Check that ack isn't -1; it's spurious
Set priority using pkts, not bytes, dynamically update priority to reflect
actual number of bytes left in flow (idea from pFabric SIGCOMM 2014)
More accurate implementation of SRPT => use starvation-prevention, as suggested by pFabric.
checkinterval is now 1 ms to update priority more frequently, ideally we want to update priority on every ack using Two Way TCP, but it seems to be too much work for now.