!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 have a bad feeling about this: I tried wrapping in gålbma for conveying extra information (have not yet checked what you use this for here) but it turned out to be hairy and brittle with all the unwrapping and rewrapping that became necessary over time, so I just ripped it out and now I am much happier.