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-cluster/src/multi-jvm/scala/akka/cluster/RestartNodeSpec.scala

@@ -23,7 +23,7 @@ import akka.remote.testkit.MultiNodeConfig
 import akka.remote.testkit.MultiNodeSpec
 import akka.testkit._
 import com.typesafe.config.ConfigFactory
-import akka.util.ccompat.imm._
+import akka.util.ccompat._
 
 object RestartNodeMultiJvmSpec extends MultiNodeConfig {
   val first = role("first")