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>
MB-12750 proxy remote cluster rest api to goxdcr

the following api's

- GET  /pools/default/remoteClusters
- POST /pools/default/remoteClusters
- POST /pools/default/remoteCluster/Id
- DELETE  /pools/default/remoteCluster/Id

Change-Id: I49ec3039397912793b113e478b5e45d3fd10bacb
Reviewed-on: http://review.couchbase.org/44296
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-12750 proxy xdcr replications rest api to goxdcr

the following api's

- GET /settings/replications
- GET /settings/replications/replication_id
- POST /settings/replications
- POST /settings/replications/replication_id
- POST /controller/createReplication
- POST /controller/cancelXDCR/replication_id
- DELETE /controller/cancelXDCR/replication_id

Change-Id: I921f64c642fa63cba2544129e359b3da645bc308
Reviewed-on: http://review.couchbase.org/44297
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-12750 handle potential "cancelXCDR" typo in request path

this was original typo in rest api's and potentially some clients
still use this entry point

Change-Id: Ibb0e0198a68292e5f0c8de9a8d298e65573fc9df
Reviewed-on: http://review.couchbase.org/44298
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>