MB-12210: log xdcr dcp activity to xdcr log

Change-Id: Ida60e2f2910463d93db25a9887e97fb9966cc0da
Reviewed-on: http://review.couchbase.org/42529
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>