Add a getNumRemainingStatusRequests() function to SessionImpl to get the current integer as it decreases

[nmontana42]

I'm interested in creating a PriorityQueue containing n sessions, prioritized by the remaining status requests. The queue would provide a session that is most likely to be configured and open. At the moment there doesn't seem to be public method that obtains that integer. I believe the closest thing is statically referencing the MAX_STATUS_RETRY int declared inside SessionImpl

This seems more robust than using the most recent timestamp