backup: debounce fine-grained backup (#1036)

[ti-srebot]

cherry-pick #1036 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/1037

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

git push [email protected]:ti-srebot/br.git pr/1037:release-5.0-55f18432033b

---

What problem does this PR solve?

Sometimes, TiKV make empty response stream from backing up request. (This seems buggy, but not sure.)

In this condition, fine-grained backup would crazily send backup RPC, if there are enough resource, thing wouldn't be so bad: when PD make progress, real leader can be found, and everything should back to normal, but for our CI, the busiest node of our community, un, ya see.

Note the timestamp. There are about 100k+ or even more logs like this.

What is changed and how it works?

Now, if fine-grained backup make no progress, it would backoff for 10 seconds.
The backoff time is an experience value, by analyzing the local log. Let's change it if necessary.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Release Note

• Fix a bug that caused BR send too many useless RPCs to TiKV.

[glorv]

/lgtm

[kennytm]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[kennytm]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 714d1ca

[ti-chi-bot]

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[3pointer]

/merge