Deprecate constructors

We need to change constructors to add new things, like location and properties on DatasetDescriptors. But, we have to deprecate and support old constructors. This isn't always possible. For example, adding a required location isn't backward-compatible. Optional additions can be made backwards-compatible, but require adding new constructors.

I'd like to deprecate constructors in favor of builders and URI-based instantiation. To support Guice/Spring instantiation, we should keep only the most general constructor public, and note in docs that it may change and is for injection---because Guice won't care if it changes as long as it has a binding.