CBQE-1100: periodically report kv op latency

Every 10 seconds gather kv op latency (see app/config.py)
latancy collected against 'default' bucket but to be configurable by test-json

Change-Id: Ic68a35bd948c9ff72510f48f1d1fdf9a951e4dc0
Reviewed-on: http://review.couchbase.org/25119
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>