CDK-639: Qualify DatasetDescriptor schema paths.

This uses the schema URI rather than schema URL where possible and adds
an accessor for the URI. This also replaces new URI(...) with URI.create
and eliminates the URISyntaxExceptions that were thrown.

I've tested this on the CDH5.1 quickstart VM and it's working with both Hive and Impala.
No need to make this public. In fact, would it be possible to avoid having this method and use getSchemaUrl everywhere? URLs can be retrieved, whereas URIs do not offer that guarantee (e.g. there may not be a protocol handler for the URI).