scheduler: speed up exit

[han-ian]

What problem does this PR solve?

Issue Number: Close #5274

What is changed and how does it work?

     if coordinator's ctx has been closed, don't do more schedule. 

Check List

Tests

• Manual test (add detailed scripts or steps below)
  For our cluster with more than 2000 tikv-servers, pd transfer leader can be done in 1 second.

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

speed up scheduler exit  

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