detach concurrency_throttle from xdc_replicator.hrl

Because there's nothing in this code that requires xdc_replicator.hrl

As part of this work I've moved concurrency_throttle_state record to
concurrency_throttle module because there's no need to define it in
include file.

Change-Id: I3c10fa2b1b837c872282c7910bcaade3fa555126
Reviewed-on: http://review.couchbase.org/37815
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>