Implement circuit binding and start on intro-point establisher logic

This branch rebases earlier work from !1465 (closed) to incorporate necessary preliminaries, and @Diziet's comments. It's about as far as I think I will be able to advance this before I go on break. (Conceivably I'll add an extra commit I guess.)

Although the tor-hsservice part is not complete, I think that should not block a merge if you think the rest of this is ready: it moves things forward and refines our APIs a little.

Closes #993 (closed).

Starts on #976 (closed).