CBQE-789: store query response time into seriesly

Change-Id: Id48daf9a94fab528e6e5591361d5cf98059bf01e
Note: an update is also required to cbtop to plot results
Reviewed-on: http://review.couchbase.org/25041
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>