CBQE-1100: collect memcached op latencies

Change to support latency collection in system testing.
adding new op_latency method which makes call to
underlying sdk and expects latency in return (seconds).
sdk independant.

Change-Id: I7554cca46de437d707da02eea5f888d4818cd510
Reviewed-on: http://review.couchbase.org/25116
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>