[POC] Make query constnats a final class

I am having to serialize allowed query operators to strings and back again, and I notice that PHPCR has classes for constants, e.g. `PropertyType`, but for some reason the class which has the query constants is an interface.

Is there a reason for this?

In particular I want to have access to a `nameFromValue` method as with `PropertyType` and `OnParentVersion`.