!htp #16162 Upgrade server- and client-side HTTP APIs to "final style"

This PR clears #16162.
It currently builds on #16383, so ignore Björn's commit in your review.

As a temporary measure I have added a stub TCP API as described by #16166. This `TempTcp` stuff will go away when #16166 is cleared.
>  I intentionally put there in order to make it possible to learn the real port number if the original bind request was for port zero

Ah, yes. Good point! I'll try to get that in right away.
I am currently working on another iteration which will be based on the "composing things in a trait" approach.
Should be done tonight.
I'll ping here once done.
Thanks!