+str #15086 subscription-timeout for publishers in streamsofstreams

By cancelling Subscribers and Publishers, not actors directly etc. In theory anyone could reuse the registry - I have not made it akka private, only the internals of it.

Only in the GroupBy case currently. For it it works because when all substreams are cancelled the entire actor stops itself.

I avoided even thinking about any WeakReference magics, I believe we agreed to avoid them right?

Anyhow, still WIP as needs to be applied to other places which might need it, but comments on impl method already very welcome, thanks!

Eventually resolves #15086
/** WARNING: Not A General Purpose ExecutionContext!
   *
   * This is an execution context which runs everything on the calling thread.
   * It is very useful for actions which are known to be non-blocking and
   * non-throwing in order to save a round-trip to the thread pool.
*/