[SPARK-6059][Yarn] Add volatile to ApplicationMaster's reporterThread and allocator

[zsxwing]

ApplicationMaster.reporterThread and ApplicationMaster.allocator are accessed in multiple threads, so they should be marked as volatile.

[SparkQA]

Test build #28063 has started for PR 4814 at commit 17d9386.

• This patch merges cleanly.

[srowen]

I agree. LGTM pending tests.

[SparkQA]

Test build #28063 has finished for PR 4814 at commit 17d9386.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28063/
Test PASSed.

[srowen]

I'm going to merge into master only just because it's not clear there is an actual practical issue as a result of this.