[cherry pick] pd backoff use tikv.rawkv.default_backoff_in_ms (#288)

[marsishandsome]

Signed-off-by: marsishandsome marsishandsome@gmail.com

What problem does this PR solve?

cherry pick #288

What is changed and how it works?

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

[marsishandsome]

/run-all-tests

[birdstorm]

LGTM