MB-12750 partially forwarding /internalSettings requsts to goxdcr

- GET retrieve values from goxdcr and merge them with ns_server internal settings
- POST process all settings known to ns_server and forward the rest of them to
  goxdcr

Change-Id: I0f21214f035323795b166f64868e06e5ad26e0d7
Reviewed-on: http://review.couchbase.org/44258
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>