pillowfight should build histogram for logical operations

pillowfight's timings seem to be reporting the latency of the initial operation rather than the total time taken to satisfy the durability constraints.

e.g. I forced pillowfight to check for persistence only once every 100ms. This gave me a steady 10 writes / sec on the cluster. My expectation was that the pillowfight timings would all show latencies around the 100ms mark - instead there's a bunch under 1ms and the highest is 44ms.
Command used to stimulate this behaviour was:

cbc-pillowfight -U couchbase://10.142.175.101/charlie -u Administrator -P password -I 10000 -t1 -B1 -m 1024 -M 1024 -Ddurability_interval=1 --persist-to=2 -T --set-pct=100