SPARK-2282: Reuse PySpark Accumulator sockets to avoid crashing Spark

JIRA: https://issues.apache.org/jira/browse/SPARK-2282

This issue is caused by a buildup of sockets in the TIME_WAIT stage of TCP, which is a stage that lasts for some period of time after the communication closes.

This solution simply allows us to reuse sockets that are in TIME_WAIT, to avoid issues with the buildup of the rapid creation of these sockets.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1220 from aarondav/SPARK-2282 and squashes the following commits:

2e5cab3 [Aaron Davidson] SPARK-2282: Reuse PySpark Accumulator sockets to avoid crashing Spark

core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala

@@ -599,6 +599,8 @@ private class PythonAccumulatorParam(@transient serverHost: String, serverPort:
     } else {
       // This happens on the master, where we pass the updates to Python through a socket
       val socket = new Socket(serverHost, serverPort)
+      // SPARK-2282: Immediately reuse closed sockets because we create one per task.
+      socket.setReuseAddress(true)
       val in = socket.getInputStream
       val out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream, bufferSize))
       out.writeInt(val2.size)