Disable xdcr trace logger by default.

Change-Id: If3b442c21bb163f521ccb4f87e6c6982d6174014
Reviewed-on: http://review.couchbase.org/36603
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>