=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-cluster/src/test/scala/akka/cluster/StartupWithOneThreadSpec.scala

@@ -29,8 +29,7 @@ object StartupWithOneThreadSpec {
     akka.actor.default-dispatcher {
       executor = thread-pool-executor
       thread-pool-executor {
-        core-pool-size-min = 1
-        core-pool-size-max = 1
+        fixed-pool-size = 1
       }
     }
     """