CDK-140: Add namespaces

Yes, datasets created before namespaces were introduced will use the same URIs. The only URIs that will no longer work are the ones for datasets that were in the root of the file system, like "dataset:hdfs:/events". I think it's reasonable to expect that there aren't any because permissions almost always disallow users creating new directories in /. In those cases, the user will have to use the SPI to open old datasets.