[close #663] Select TiFlash Stores Round-Robin

[shiyuhang0]

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

[codecov]

Codecov Report

Base: 34.81% // Head: 35.06% // Increases project coverage by +0.25% 🎉

Coverage data is based on head (841c298) compared to base (1b5edcd).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #662      +/-   ##
============================================
+ Coverage     34.81%   35.06%   +0.25%     
- Complexity     1433     1446      +13     
============================================
  Files           278      278              
  Lines         17377    17380       +3     
  Branches       1974     1975       +1     
============================================
+ Hits           6049     6095      +46     
+ Misses        10715    10666      -49     
- Partials        613      619       +6     

Impacted Files	Coverage Δ
...ain/java/org/tikv/common/region/RegionManager.java	79.73% <80.00%> (+5.73%)	⬆️
...rc/main/java/io/grpc/netty/NettyClientHandler.java	57.11% <0.00%> (+0.21%)	⬆️
src/main/java/io/grpc/internal/ClientCallImpl.java	56.82% <0.00%> (+0.48%)	⬆️
src/main/java/io/grpc/stub/ClientCalls.java	48.51% <0.00%> (+2.31%)	⬆️
...va/org/tikv/common/region/StoreHealthyChecker.java	72.15% <0.00%> (+2.53%)	⬆️
...ty/handler/codec/http2/Http2ConnectionHandler.java	52.07% <0.00%> (+3.91%)	⬆️
src/main/java/org/tikv/common/region/TiRegion.java	73.50% <0.00%> (+4.27%)	⬆️
.../main/java/org/tikv/common/region/TiStoreType.java	83.33% <0.00%> (+16.66%)	⬆️

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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.

[iosmanthus]

LGTM

[iosmanthus]

LGTM

[ti-srebot]

cherry pick to release-3.3 in PR #688