Third-party registration - code and UTs for forwarding REGISTERs on

This isn't complete - there are TODO comments, and I want to rework the handling of Expires headers for multiple bindings a bit - but I think I'm far enough along that it's worth reviewing.

I don't have support for deregistration yet, but I expect that to mostly just involve calling register_with_application_servers with an expires parameter of 0.