tor-hssvc: wip IPT Establisher

This is a very very initial stab at one of the APIs implied by !1429 (merged). Insofar relevant, it is intended to take into account #966.

I have created it in tor-hsservice/src, but there's no mod statement and it wouldn't compile if there were.

This is a convenient way to transfer my WIP work to Nick. I don't ncecessarily expect the final result to look very much like this, but these are the thoughts I had so far.

@nickm, feel free to steal this branch and close the MR, or merge it and work on it in-tree, whichever you think seems most convenient.