[SPARK-10125] [STREAMING] Fix a potential deadlock in JobGenerator.stop

Because `lazy val` uses `this` lock, if JobGenerator.stop and JobGenerator.doCheckpoint (JobGenerator.shouldCheckpoint has not yet been initialized) run at the same time, it may hang.

Here are the stack traces for the deadlock:

```Java
"pool-1-thread-1-ScalaTest-running-StreamingListenerSuite" #11 prio=5 os_prio=31 tid=0x00007fd35d094800 nid=0x5703 in Object.wait() [0x000000012ecaf000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        at java.lang.Thread.join(Thread.java:1245)
        - locked <0x00000007b5d8d7f8> (a org.apache.spark.util.EventLoop$$anon$1)
        at java.lang.Thread.join(Thread.java:1319)
        at org.apache.spark.util.EventLoop.stop(EventLoop.scala:81)
        at org.apache.spark.streaming.scheduler.JobGenerator.stop(JobGenerator.scala:155)
        - locked <0x00000007b5d8cea0> (a org.apache.spark.streaming.scheduler.JobGenerator)
        at org.apache.spark.streaming.scheduler.JobScheduler.stop(JobScheduler.scala:95)
        - locked <0x00000007b5d8ced8> (a org.apache.spark.streaming.scheduler.JobScheduler)
        at org.apache.spark.streaming.StreamingContext.stop(StreamingContext.scala:687)

"JobGenerator" #67 daemon prio=5 os_prio=31 tid=0x00007fd35c3b9800 nid=0x9f03 waiting for monitor entry [0x0000000139e4a000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at org.apache.spark.streaming.scheduler.JobGenerator.shouldCheckpoint$lzycompute(JobGenerator.scala:63)
        - waiting to lock <0x00000007b5d8cea0> (a org.apache.spark.streaming.scheduler.JobGenerator)
        at org.apache.spark.streaming.scheduler.JobGenerator.shouldCheckpoint(JobGenerator.scala:63)
        at org.apache.spark.streaming.scheduler.JobGenerator.doCheckpoint(JobGenerator.scala:290)
        at org.apache.spark.streaming.scheduler.JobGenerator.org$apache$spark$streaming$scheduler$JobGenerator$$processEvent(JobGenerator.scala:182)
        at org.apache.spark.streaming.scheduler.JobGenerator$$anon$1.onReceive(JobGenerator.scala:83)
        at org.apache.spark.streaming.scheduler.JobGenerator$$anon$1.onReceive(JobGenerator.scala:82)
        at org.apache.spark.util.EventLoop$$anon$1.run(EventLoop.scala:48)
```

I can use this patch to produce this deadlock: zsxwing@8a88f28

And a timeout build in Jenkins due to this deadlock: https://amplab.cs.berkeley.edu/jenkins/job/NewSparkPullRequestBuilder/1654/

This PR initializes `checkpointWriter` before `eventLoop` uses it to avoid this deadlock.

Author: zsxwing <zsxwing@gmail.com>

Closes #8326 from zsxwing/SPARK-10125.

streaming/src/main/scala/org/apache/spark/streaming/scheduler/JobGenerator.scala

@@ -79,6 +79,10 @@ class JobGenerator(jobScheduler: JobScheduler) extends Logging {
   def start(): Unit = synchronized {
     if (eventLoop != null) return // generator has already been started
 
+    // Call checkpointWriter here to initialize it before eventLoop uses it to avoid a deadlock.
+    // See SPARK-10125
+    checkpointWriter
+
     eventLoop = new EventLoop[JobGeneratorEvent]("JobGenerator") {
       override protected def onReceive(event: JobGeneratorEvent): Unit = processEvent(event)