[SPARK-19777] Scan runningTasksSet when check speculatable tasks in TaskSetManager.

[jinxing64]

What changes were proposed in this pull request?

When check speculatable tasks in TaskSetManager, only scan runningTasksSet instead of scanning all taskInfos.

How was this patch tested?

Existing tests.

[jinxing64]

cc @kayousterhout @squito

[squito]

lgtm

(btw, I just meant to update your old PR to mention the new jira in the title -- you didn't have to open a new pr. but this works too)

[jinxing64]

@squito
Thanks a lot :)

[SparkQA]

Test build #73649 has finished for PR 17111 at commit 819a5e8.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #73667 has finished for PR 17111 at commit 6840ce1.

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

[kayousterhout]

Thanks for fixing this! I've merged this.

[jinxing64]

@kayousterhout
Thanks for merging.
(btw, I made some measurements for #16867 SPARK-16929, please take a look when you have time :) )