!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.
They are the same type, yes, but this means that I have to think about it even more!
Because I don't know which is input and which is output.
The `flow` of an `IncomingConnection` only really represents the client if I send bytes "through" the client with the server being the one pushing (I'm avoiding the word "active" here :). In the case of an HTTP client for example this `flow` is not really an actual flow, just a disconnected pair of a Source and a Sink, which is why it makes no sense to surface it.
This disparity between clients that I (as a server) can really push things through (the `flow` makes sense) and clients for which I can't (the `flow` doesn't make sense) is what we are fighting with here.