!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 realized that my comment wasn't that clear. I like the `handleWith` as an entry level easy API. Now we should only get the rest of the machinery to accept `conn handleWith Flow.empty`.