Implement rendezvous setup, and hsconn attempt/retry framework

This provides actual functionality to establish an rendezvous point. And it provides the error handling regime for HS connections. (This is rather complex, because HS connection error attribution is complex.)

There is framework for adding more code.

I have not executed this code at all (!) and there are no unit tests for the new functionality, but I think the shape is about right. I think we probably want this into tree, even so, so that we can parallelise extending, testing, and bugfixing it.