[CORE] SPARK-2640: In "local[N]", free cores of the only executor should be touched by "spark.task.cpus" for every finish/start-up of tasks.

Make spark's "local[N]" better.
In our company, we use "local[N]" in production. It works exellentlly. It's our best choice.

Author: woshilaiceshide <woshilaiceshide@qq.com>

Closes #1544 from woshilaiceshide/localX and squashes the following commits:

6c85154 [woshilaiceshide] [CORE] SPARK-2640: In "local[N]", free cores of the only executor should be touched by "spark.task.cpus" for every finish/start-up of tasks.

Author: woshilaiceshide

core/src/main/scala/org/apache/spark/scheduler/local/LocalBackend.scala

@@ -57,7 +57,7 @@ private[spark] class LocalActor(
     case StatusUpdate(taskId, state, serializedData) =>
       scheduler.statusUpdate(taskId, state, serializedData)
       if (TaskState.isFinished(state)) {
-        freeCores += 1
+        freeCores += scheduler.CPUS_PER_TASK
         reviveOffers()
       }
 
@@ -68,7 +68,7 @@ private[spark] class LocalActor(
   def reviveOffers() {
     val offers = Seq(new WorkerOffer(localExecutorId, localExecutorHostname, freeCores))
     for (task <- scheduler.resourceOffers(offers).flatten) {
-      freeCores -= 1
+      freeCores -= scheduler.CPUS_PER_TASK
       executor.launchTask(executorBackend, task.taskId, task.name, task.serializedTask)
     }
   }