Add an option to allow hphp's request queue dynamically switch between FIFO and
LIFO. By default everything is still FIFO like today. Setting
ServerThreadJobLIFO to true turns everything to LIFO like today. If
ServerThreadJobLIFOThreshold is also set, then we do FIFO up until the queue
length hits the threshold. Once the threshold is crossed, workers will take
quests from the end of the queue. Once the queue size shrink below the
threshold, we resume FIFO order.
This behavior helps us prioritize newer requests when the server is loaded.
Stephen Chen