!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.
> The scaladoc states that a "passive server" scenario is one where the client initiates the conversation after the connection has been established. The doc on activeFlow describes an "active server" scenario as one where the server starts the communication after the client has connected.

This active/passive distinction (at least in the TCP case) breaks once you start to layer things. A server might have been passive at the top handler level, but now you inject another layer which does its own protocol (maybe does a handshake), and now it is not passive at all, although it is still passive at the top layer. Maybe its just me, but I don't see any scenario where I would use the handleWith method.

> When you look at the HTTP API then handleWith is the only option for handling a connection,

But *that* is fine, since it is the only meaningful way to use it. No duplication, no confusion which API should I use, since there is only one.