Collections: REST "get" to return full manifest

For the purposes of a client understanding if a KV "unknown collection" error is a hard error (the collection really doesn't exist) vs a temporary error (client is possibly racing with a REST API create), the REST API ideally needs to return the full manifest when a GET manifest call, SDK clients can then decide how to handle KV saying a collection is unknown.

Or alternatively we could implement a more targeted "GET_CID" REST API that allows "scope.collectionname" lookup from ns_server, KV implements a command that does this, but works on it's view of collections, which can be stale (hence the need to still ultimately ask ns_server).

https://docs.google.com/document/d/1X-v8GWQjplrMMaYwwWOzEuP4AUoDNIAvS39NmEjQ3_E/edit#heading=h.cv2y68pg8w25

cc Michael Nitschinger