!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.
> This active/passive distinction (at least in the TCP case) breaks once you start to layer things.

Someone will have to start the conversation and I'd argue that in the very large majority of cases the protocol makes it very clear, whether it is the server or the client who starts the conversation. (In most cases it will probably be the client.)
This question (who starts) is the single discriminator between the active and passive scenarios described above.
And the answer to this question determines, whether you use the flow directly or whether you have to `join` it.
No?