[SPARK-14316][SQL]StateStoreCoordinator should extend ThreadSafeRpcEndpoint

[zsxwing]

What changes were proposed in this pull request?

RpcEndpoint is not thread safe and allows multiple messages to be processed at the same time. StateStoreCoordinator should use ThreadSafeRpcEndpoint.

How was this patch tested?

Existing unit tests.

[SparkQA]

Test build #54670 has finished for PR 12100 at commit b949883.

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

[tdas]

Going to run multiple tests on this. LGTM otherwise.

[andrewor14]

deep-review this please

[DeepSparkBot]

@andrewor14 Review complete. LGTM.

[zsxwing]

@andrewor14 Review complete. LGTM.

Cool!

[SparkQA]

Test build #2725 has finished for PR 12100 at commit b949883.

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

[SparkQA]

Test build #2726 has finished for PR 12100 at commit b949883.

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

[SparkQA]

Test build #2724 has finished for PR 12100 at commit b949883.

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

[zsxwing]

Since all tests passed, I'm going to merge to master. Thanks, @tdas @andrewor14 and @DeepSparkBot :)