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>
MB-11435: switch to new concurrency throttle

There is now ns_config variable that allows us to pick old concurrency
throttle.

You can choose old concurrency throttle by /diag/eval-ing this:

ns_config:set(xdcr_use_old_throttle, true).

Change-Id: I194ce4b6e7d22cd8d573913ec2c292a966c3bbbe
Reviewed-on: http://review.couchbase.org/39192
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>