Make Executor fields volatile since they're read from the thread pool.

A few times now I've had clusters fail due to stack trackers like below, where Kryo can't load my spark.kryo.registrator class. It's really odd because sometimes it works, sometimes it doesn't.

Looking at the code, I'm wondering if Executor:89:

```
      Thread.currentThread.setContextClassLoader(urlClassLoader)
```

Was reading a stale value (null), and so the context class loader wasn't being set.

I'm running this patch on some jobs now to see if it helps, but the failure is random, so just a few good jobs isn't a guarantee. But either way, it seems like these fields are access in a multithreaded way, so should be marked as volatile.

Right?

```
13/02/28 07:35:44 INFO cluster.TaskSetManager: Loss was due to java.lang.ClassNotFoundException: com.MyRegistrar
	at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:247)
	at spark.KryoSerializer.createKryo(KryoSerializer.scala:209)
	at spark.KryoSerializer$$anon$1.initialValue(KryoSerializer.scala:111)
	at spark.KryoSerializer$$anon$1.initialValue(KryoSerializer.scala:110)
	at java.lang.ThreadLocal.setInitialValue(ThreadLocal.java:141)
	at java.lang.ThreadLocal.get(ThreadLocal.java:131)
	at spark.KryoSerializer.newInstance(KryoSerializer.scala:216)
	at spark.broadcast.HttpBroadcast$.read(HttpBroadcast.scala:127)
	at spark.broadcast.HttpBroadcast.readObject(HttpBroadcast.scala:40)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
	at scala.collection.immutable.$colon$colon.readObject(List.scala:435)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
	at scala.collection.immutable.$colon$colon.readObject(List.scala:435)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
	at scala.collection.immutable.$colon$colon.readObject(List.scala:435)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
	at spark.JavaDeserializationStream.readObject(JavaSerializer.scala:23)
	at spark.scheduler.ResultTask$.deserializeInfo(ResultTask.scala:42)
	at spark.scheduler.ResultTask.readExternal(ResultTask.scala:105)
	at java.io.ObjectInputStream.readExternalData(ObjectInputStream.java:1791)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1750)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
	at spark.JavaDeserializationStream.readObject(JavaSerializer.scala:23)
	at spark.JavaSerializerInstance.deserialize(JavaSerializer.scala:45)
	at spark.executor.Executor$TaskRunner.run(Executor.scala:98)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:662)
```