Make AllocatedPersistentTask members volatile (elastic#2297)

These members are default initialized on contruction and then set by the
init() method.  It's possible that another thread accessing the object
after init() is called could still see the null/0 values, depending on how
the compiler optimizes the code.

server/src/main/java/org/elasticsearch/persistent/AllocatedPersistentTask.java

@@ -36,16 +36,16 @@
  * Represents a executor node operation that corresponds to a persistent task
  */
 public class AllocatedPersistentTask extends CancellableTask {
-    private String persistentTaskId;
-    private long allocationId;
+    private volatile String persistentTaskId;
+    private volatile long allocationId;
 
     private final AtomicReference<State> state;
     @Nullable
-    private Exception failure;
+    private volatile Exception failure;
 
-    private PersistentTasksService persistentTasksService;
-    private Logger logger;
-    private TaskManager taskManager;
+    private volatile PersistentTasksService persistentTasksService;
+    private volatile Logger logger;
+    private volatile TaskManager taskManager;
 
 
     public AllocatedPersistentTask(long id, String type, String action, String description, TaskId parentTask) {