=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-actor/src/main/scala/akka/dispatch/ThreadPoolBuilder.scala

@@ -123,17 +123,14 @@ final case class ThreadPoolConfigBuilder(config: ThreadPoolConfig) {
   def withNewThreadPoolWithArrayBlockingQueueWithCapacityAndFairness(capacity: Int, fair: Boolean): ThreadPoolConfigBuilder =
     this.copy(config = config.copy(queueFactory = arrayBlockingQueue(capacity, fair)))
 
+  def setFixedPoolSize(size: Int): ThreadPoolConfigBuilder =
+    this.copy(config = config.copy(corePoolSize = size, maxPoolSize = size))
+
   def setCorePoolSize(size: Int): ThreadPoolConfigBuilder =
-    if (config.maxPoolSize < size)
-      this.copy(config = config.copy(corePoolSize = size, maxPoolSize = size))
-    else
-      this.copy(config = config.copy(corePoolSize = size))
+    this.copy(config = config.copy(corePoolSize = size, maxPoolSize = math.max(size, config.maxPoolSize)))
 
   def setMaxPoolSize(size: Int): ThreadPoolConfigBuilder =
-    if (config.corePoolSize > size)
-      this.copy(config = config.copy(corePoolSize = size, maxPoolSize = size))
-    else
-      this.copy(config = config.copy(maxPoolSize = size))
+    this.copy(config = config.copy(maxPoolSize = math.max(size, config.corePoolSize)))
 
   def setCorePoolSizeFromFactor(min: Int, multiplier: Double, max: Int): ThreadPoolConfigBuilder =
     setCorePoolSize(scaledPoolSize(min, multiplier, max))