Internal dispatcher to protect against starvation (#26816)

* Allow for dispatcher aliases and define a internal dispatcher
* Test checking dispatcher name
* MiMa for Dispatchers
* Migration guide entry
* No need to have custom dispatcher lookup logic in streams anymore
* Default dispatcher size and migration note about that
* Test checking exact config values...
* Typed receptionist on internal dispatcher
* All internal usages of system.dispatcher gone through

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

@@ -80,10 +80,7 @@ object ReplicatorSettings {
    * the default configuration `akka.cluster.distributed-data`.
    */
   def apply(config: Config): ReplicatorSettings = {
-    val dispatcher = config.getString("use-dispatcher") match {
-      case "" => Dispatchers.DefaultDispatcherId
-      case id => id
-    }
+    val dispatcher = config.getString("use-dispatcher")
 
     val pruningInterval = toRootLowerCase(config.getString("pruning-interval")) match {
       case "off" | "false" => Duration.Zero
@@ -299,7 +296,7 @@ final class ReplicatorSettings(
 
   def withDispatcher(dispatcher: String): ReplicatorSettings = {
     val d = dispatcher match {
-      case "" => Dispatchers.DefaultDispatcherId
+      case "" => Dispatchers.InternalDispatcherId
       case id => id
     }
     copy(dispatcher = d)