MB-100: added facility to trace latency histograms

Change-Id: I39cbf7094b1cbee9e5f0e74bbda4984e35369a19
Reviewed-on: http://review.couchbase.org/30145
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>