Introduce warning silencer plugin (#26588)

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

akka-distributed-data/src/main/scala/akka/cluster/ddata/VersionVector.scala

@@ -386,5 +386,5 @@ final case class ManyVersionVector(versions: TreeMap[UniqueAddress, Long]) exten
     else this
 
   override def toString: String =
-    versions.map { case ((n, v)) => n + " -> " + v }.mkString("VersionVector(", ", ", ")")
+    versions.map { case ((n, v)) => n.toString + " -> " + v }.mkString("VersionVector(", ", ", ")")
 }