MB-8663 Add REST APIs for working with global xdcr settings.

For consistency. Supported parameters are all the same plus
enableExtraDebug.

Change-Id: I632ba27de5ab2a5ffe19c4ad675e66604accc68f
Reviewed-on: http://review.couchbase.org/28182
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>