Decide between URI and URL for descriptors

We recently added DatasetDescriptor#getUri, but need to follow up and decide whether URI or URL is correct to expose through the API. URL is easier to open, but Java URLs have requires a hacky handler addition in Kite for hdfs URIs. This handler calls new Configuration() (which should be updated for DefaultConfiguration) and is used for any URI that had no handler. So while the API guarantees there is a handler, there is no guarantee that the URL's openStream method will work without throwing an exception from Hadoop's FileSystem.