[SPARK-6602][Core]Remove unnecessary synchronized

[zsxwing]

A follow-up pr to address #5392 (comment)

[AmplabJenkins]

Merged build triggered.

[AmplabJenkins]

Merged build started.

[SparkQA]

Test build #36222 has started for PR 7141 at commit fcf7b50.

[rxin]

lgtm

[SparkQA]

Test build #36222 has finished for PR 7141 at commit fcf7b50.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class Heartbeat(workerId: String, worker: RpcEndpointRef) extends DeployMessage
  • case class RegisteredWorker(master: RpcEndpointRef, masterWebUiUrl: String) extends DeployMessage
  • case class RegisterApplication(appDescription: ApplicationDescription, driver: RpcEndpointRef)
  • case class RegisteredApplication(appId: String, master: RpcEndpointRef) extends DeployMessage
  • case class SubmitDriverResponse(
  • case class KillDriverResponse(
  • case class MasterChanged(master: RpcEndpointRef, masterWebUiUrl: String)

[AmplabJenkins]

Merged build finished. Test PASSed.