CA-147261: Add average queue size metric for tapdisk3

Prior to this patch, the average queue size metric was missing.
This patch computes this metric using the stats exposed by tapdisk3.
Average queue size is defined as the sum of the average read and write queues.
Additionally, we maintain two variables to retain the previous values of
this metric.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
@franciozzy Hi, thanks for looking into this but my intention was to combine both rd and wr into a single datasource. I've computed read (`avg_rd_qu_sz`) and write (`avg_wr_qu_sz`) separately and added them together to represent avgqu_sz. 
Please correct me if I am wrong.