TiSession support graceful close (#238)

[ti-srebot]

cherry-pick #238 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/243

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

git push git@github.com:ti-srebot/client-java.git pr/243:release-3.1-257f3062c4d9

---

What problem does this PR solve?

TiSession support graceful close

What is changed and how it works?

Current TiSession.close call ExecutorService.shutdownNow directly.

Add a new function TiSession.closeAwaitTermination:

1. call ExecutorService.shutdown
2. check status using ExecutorService.isTerminated, if all terminated return
3. if timeout, call ExecutorService.shutdown

Check List

Tests

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

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