+str #15904 Ported Tcp IO to new Stream DSL

* Port of Tcp IO to the new Stream DSL
* ~~Ordered shutdown of the server is to be discussed (was missing in the old one as well)~~
* ~~This PR also includes the broadcast fixes for #15948 which can be ignored (will be removed)~~

Fixes #15904
I see. However, on the client side we now don't have that problem any more, because I specify the Source and Sink in the `Connect` message and the IO layer will materialize it.
Anyway, I'm just trying to really understand the new API, not saying that anything is wrong here.