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/ORSet.scala

@@ -388,8 +388,9 @@ final class ORSet[A] private[akka] (
 
   /**
    * Removes all elements from the set, but keeps the history.
-   * This has the same result as using [[#remove]] for each
-   * element, but it is more efficient.
+   * This has the same result as using
+   * [[ORSet#remove(node:akka\.cluster\.ddata\.SelfUniqueAddress*]]
+   * for each element, but it is more efficient.
    */
   def clear(@unused node: SelfUniqueAddress): ORSet[A] = clear()