So now we can compile akka-distributed-data with
-Xfatal-warnings - though I'm not yet sure about
enabling the (other) undisciplineScalacOptions
* Fix multi-node silencing
* Fix scaladoc warnings
* Introduce annotation to declare ccompat use
* Add explicit toString
* Fix deprecation on 2.13
* Move 'immutable' ccompat helpers to shared ccompat package
* Add MiMa for internal scala 2.13 compatibility class
* Internal API markers
* Fix scaladoc generation
Got bitten by https://github.com/scala/bug/issues/11021
d390fcf183