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-cluster-metrics/src/main/resources/reference.conf

@@ -22,8 +22,7 @@
 # Provides periodic statistics collection and publication throughout the cluster.
 akka.cluster.metrics {
   # Full path of dispatcher configuration key.
-  # Use "" for default key `akka.actor.default-dispatcher`.
-  dispatcher = ""
+  dispatcher = "akka.actor.default-dispatcher"
   # How long should any actor wait before starting the periodic tasks.
   periodic-tasks-initial-delay = 1s
   # Sigar native library extract location.

akka-cluster-metrics/src/main/scala/akka/cluster/metrics/ClusterMetricsSettings.scala

@@ -5,7 +5,6 @@
 package akka.cluster.metrics
 
 import com.typesafe.config.Config
-import akka.dispatch.Dispatchers
 import scala.concurrent.duration.FiniteDuration
 import akka.util.Helpers.Requiring
 import akka.util.Helpers.ConfigOps
@@ -19,10 +18,7 @@ case class ClusterMetricsSettings(config: Config) {
   private val cc = config.getConfig("akka.cluster.metrics")
 
   // Extension.
-  val MetricsDispatcher: String = cc.getString("dispatcher") match {
-    case "" => Dispatchers.DefaultDispatcherId
-    case id => id
-  }
+  val MetricsDispatcher: String = cc.getString("dispatcher")
   val PeriodicTasksInitialDelay: FiniteDuration = cc.getMillisDuration("periodic-tasks-initial-delay")
   val NativeLibraryExtractFolder: String = cc.getString("native-library-extract-folder")