MB-11435: implemented new concurrency throttle

New throttle implements more or less round-robin scheduling of
workers, which is different from "least loaded destination" of
existing concurrency throttle. But new throttle is built to be faster.

Change-Id: I8577357070028a3a77f43a3913fd77562a877263
Reviewed-on: http://review.couchbase.org/39190
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>