[SPARK-5069][Core] Fix the race condition of TaskSchedulerImpl.dagScheduler

[zsxwing]

It's not necessary to set TaskSchedulerImpl.dagScheduler in preStart. It's safe to set it after initializeEventProcessActor().

[SparkQA]

Test build #25030 has started for PR 3887 at commit d95894f.

• This patch merges cleanly.

[SparkQA]

Test build #25030 has finished for PR 3887 at commit d95894f.

• 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/25030/
Test PASSed.

[rxin]

Merging in master. Thanks!