MB-12210: send xdcr dcp open_connection messages to xdcr log

Change-Id: I2320996042b6ef30c4694b982f78e4939d4be02f
Reviewed-on: http://review.couchbase.org/42533
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>