MB-10849: add stat for priority queue sizes

Unblock QE tests that validate bucket priority UI setting.
'cbstats raw workload' now lists the sizes of futureQueue
and readyQueue as Incoming Q and Outgoing Q respectively...
HiPrioQ_AuxIO:InQsize:            1
HiPrioQ_AuxIO:OutQsize:           0
HiPrioQ_NonIO:InQsize:            8
HiPrioQ_NonIO:OutQsize:           0
HiPrioQ_Reader:InQsize:           4
HiPrioQ_Reader:OutQsize:          0
LowPrioQ_Writer:OutQsize:         0
etc

Change-Id: I023b16e3cbf953eb110b5be6b9c5a630201fa1ed
Reviewed-on: http://review.couchbase.org/35913
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>