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/DurableStore.scala

@@ -243,7 +243,7 @@ final class LmdbDurableStore(config: Config) extends Actor with ActorLogging {
           dbPut(OptionVal.None, key, data)
         } else {
           if (pending.isEmpty)
-            context.system.scheduler.scheduleOnce(writeBehindInterval, self, WriteBehind)(context.system.dispatcher)
+            context.system.scheduler.scheduleOnce(writeBehindInterval, self, WriteBehind)(context.dispatcher)
           pending.put(key, data)
         }
         reply match {