MB-11435: made old concurrency_throttle to behave as new throttle

I.e. so that we can send same calls and casts to both throttle kinds
and expect things to work.

Change-Id: Ic1fba0b5a8000dff2cd29f22cc617a024f94f327
Reviewed-on: http://review.couchbase.org/39191
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>