[Reviewer: Mike] Add support for public GRUUs

This is ready for review. I've tested by:

* Running the UTs (full coverage, no leaks) and eyeballing the debug logs
* Running my new live test suite, including validating the NOTIFY XML against a schema and using a non-UUID instance-id
* Checking SAS for a call without GRUUs, a call to a GRUU, a call to a nonexistent GRUU, and a call to a GRUU where the binding failed Reject-Contact filtering, and checked that the logs are correct in each case

Should this be something like perm_gruu() to make it clear it is a permanent GRUU it returns.  At some point we'll probably want to add temporary GRUU support.