[close #375] warm up RawKVClient while creating it (#367)

[ti-srebot]

cherry-pick #367 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/381

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

git push git@github.com:ti-srebot/client-java.git pr/381:release-3.1-c95479e8ad24

---

Signed-off-by: iosmanthus myosmanthustree@gmail.com

What problem does this PR solve?

We find some initialization work that needs to do in the first gRPC call in RawKVClient which might result in a timeout in the first few gRPC calls.

First run:

Second run:

What is changed and how it works?

To solve this problem, we may launch a raw_get request to warm up the runtime.

Check List

Tests

• 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

[zz-jason]

LGTM