[close #663] Select TiFlash Stores Round-Robin (#662)

[ti-srebot]

cherry-pick #662 to release-3.3
You can switch your code base to this Pull Request by using git-extras:

# In client-java repo:
git pr https://github.com/tikv/client-java/pull/688

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/client-java.git pr/688:release-3.3-8cb7a8294c4c

---

What problem does this PR solve?

Problem Description: client-java always selects the first TiFlash store which may cause read hotspot.

What is changed and how does it work?

Choose the TiFlash stores Round-Robin

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has methods of interface change
• Has persistent data change
• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Possible performance regression, WHY: TBD
• Increased code complexity, WHY: TBD
• Breaking backward compatibility, WHY: TBD
• NO side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note
• NO related changes

[ti-srebot]

/run-all-tests

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-3.3@6c0f6ad). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@              Coverage Diff               @@
##             release-3.3     #688   +/-   ##
==============================================
  Coverage               ?   35.01%           
  Complexity             ?     1447           
==============================================
  Files                  ?      278           
  Lines                  ?    17380           
  Branches               ?     1975           
==============================================
  Hits                   ?     6086           
  Misses                 ?    10683           
  Partials               ?      611           

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[ti-srebot]

@zhangyangyu, Thanks for your review. The bot only counts LGTMs from Reviewers and higher roles, but you're still welcome to leave your comments. You are not a reviewer or committer or co-leader or leader.

[zhangyangyu]

/merge

[ti-srebot]

@zhangyangyu Oops! This PR requires at least 1 LGTMs to merge. The current number of LGTM is 0