SPARK-5370. [YARN] Remove some unnecessary synchronization in YarnAlloca...

[sryza]

...tor

[SparkQA]

Test build #25970 has started for PR 4164 at commit 0c8d736.

• This patch merges cleanly.

[tgravescs]

changes look fine, waiting on jenkins.

[SparkQA]

Test build #25970 has finished for PR 4164 at commit 0c8d736.

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

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/25970/
Test PASSed.