MB-5487: apply updated xdcr token count immediately

With this commit when max_concurrent_reps changes, we're passing
updated value to concurrency throttle without any delays.

Change-Id: I22eedd359a1d6ade697bfa9d1e7a820df2161454
Reviewed-on: http://review.couchbase.org/33707
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>