!str #16039 Remove old scaladsl, rename scaladsl2

* and impl2, testkit2
* keeping io2 for now

Are there any more things in impl2 that are not used?

One failing test (ignored for now): io2.TcpFlowSpec
See FIXME in that test. What can I have done?
I found one bug, sender used wrong, but the issue remains:

    val handler = context.actorOf(testClientProps(sender()))