MB-10073: added support for pauseRequested xdcr option

Which simply forces effective token count to drop to 0.

Change-Id: I63cda4d184c27e24f583f5a16c11291e480d93d4
Reviewed-on: http://review.couchbase.org/33709
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>