stats: disable stats by default (#693)

[ti-srebot]

cherry-pick #693 to release-5.0-rc
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/697

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

git push git@github.com:ti-srebot/br.git pr/697:release-5.0-rc-87eb3ed13336

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What problem does this PR solve?

Disable backup stats by default. because of

1. DumpStatsToJson is not stable
2. It increases memory usage may cause BR OOM

What is changed and how it works?

1. set cfg.IgnoreStats default to true
2. if cfg.IgnoreStats is true, set statsLease to -1

Check List

Tests

• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Note

• Disable backup stats by default to avoid BR reached OOM issue during backup.