!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.
Yes, I know what you are talking about.
I guess there are alternatives but all of them have their issues as well.
The core question is what we want our outside APIs to look like,
Trade-offs it is, as always...