SPARK-4300 [CORE] Race condition during SparkWorker shutdown

Close appender saving stdout/stderr before destroying process to avoid exception on reading closed input stream.
(This also removes a redundant `waitFor()` although it was harmless)

CC tdas since I think you wrote this method.

Author: Sean Owen <sowen@cloudera.com>

Closes #4787 from srowen/SPARK-4300 and squashes the following commits:

e0cdabf [Sean Owen] Close appender saving stdout/stderr before destroying process to avoid exception on reading closed input stream

core/src/main/scala/org/apache/spark/deploy/worker/ExecutorRunner.scala

@@ -85,14 +85,13 @@ private[spark] class ExecutorRunner(
     var exitCode: Option[Int] = None
     if (process != null) {
       logInfo("Killing process!")
-      process.destroy()
-      process.waitFor()
       if (stdoutAppender != null) {
         stdoutAppender.stop()
       }
       if (stderrAppender != null) {
         stderrAppender.stop()
       }
+      process.destroy()
       exitCode = Some(process.waitFor())
     }
     worker ! ExecutorStateChanged(appId, execId, state, message, exitCode)