allow parent-less concurrency_throttle

I.e. to be able to use it outside of xdcr.

Change-Id: Iba0b2fb2c731d9769273221bbf24a36b08c97d4b
Reviewed-on: http://review.couchbase.org/37818
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>