OAK-823: Missing validation for jcr:uuid property

As noted by Alex, the validator shouldn't cause code that removes and then re-adds a referenceable node to fail.
More generally, the validator should just check the consistency of the *after* state, without considering how that state was reached.

Also make the custom jcr:uuid processing only apply to mix:referenceable nodes.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1483721 13f79535-47bb-0310-9956-ffa450edef68