=act #19201 improve configuration of thread-pool-executor

* The old implementation would cap the pool size (both corePoolSize
  and maximumPoolSize) to max-pool-size, which is very confusing
  becuase maximumPoolSize is only used when the task queue is bounded.
* That resulted in configuring core-pool-size-min and core-pool-size-max
  was not enough, because it could be capped by the default max-pool-size.
* The new behavior is simply that maximumPoolSize is adjusted to not be
  less than corePoolSize, but otherwise the config properties match the
  underlying ThreadPoolExecutor implementation.
* Added a convenience fixed-pool-size property.

akka-docs/rst/java/code/docs/dispatcher/DispatcherDocTest.java

@@ -66,6 +66,15 @@ public class DispatcherDocTest {
     //#defining-dispatcher-in-code
   }
 
+  @SuppressWarnings("unused")
+  @Test
+  public void defineFixedPoolSizeDispatcher() {
+    //#defining-fixed-pool-size-dispatcher
+    ActorRef myActor = system.actorOf(Props.create(MyUntypedActor.class)
+        .withDispatcher("blocking-io-dispatcher"));
+    //#defining-fixed-pool-size-dispatcher
+  }
+
   @SuppressWarnings("unused")
   @Test
   public void definePinnedDispatcher() {