fix seekLeaderStore NPE (#366)

[ti-srebot]

cherry-pick #366 to release-3.1
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/371

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

git push git@github.com:ti-srebot/client-java.git pr/371:release-3.1-1dd7cbabe828

---

What problem does this PR solve?

This pull request fixes the NPE triggered by the double retry of seekLeaderStore. These buggy lines would also introduce another issue like peer id not match because the meta of the region is not updated.

What is changed and how it works?

Once we change detected a reachable store we need to update the peer information of the region and retry in the most outside code.

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[ti-srebot]

/run-all-tests

[zz-jason]

LGTM

[zz-jason]

LGTM

[birdstorm]

LGTM