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>