take xdcr_trace logger level from static_config

So that people can adjust it permanently too.

Change-Id: Ibd7ec159cf14b310a02e204deb6a0ed04bf42b93
Reviewed-on: http://review.couchbase.org/36610
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>