backup,restore: support backing up / restore system databases (#1048)

[ti-srebot]

cherry-pick #1048 to release-4.0
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/1077

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

git push git@github.com:ti-srebot/br.git pr/1077:release-4.0-b1ed36512074

---

What problem does this PR solve?

#872 and #679.

But this doesn't allow restore stats -- because the table IDs of the stats backed up are old table IDs, we need more efforts to rewrite them to make them take effect after the restoration.

What is changed and how it works?

This PR basically implemented #679 (comment), with some difference:

• When the target table not exists, RENAME TABLE temp.xxx TO mysql.xxx would be used, for effectively restoring user tables created in mysql schema.
• The user interface is the -f (the table filter) flag. Users must specify each table they want to restore by the table filter syntax (e.g. br restore full -f '*.*' -f '!mysql.*' -f 'mysql.usertable' to restore mysql.usertable).

Check List

Tests

• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
  • The document points out that table filter would always ignore system tables, that isn't real after this merged.
  • The usage of this feature.

Release Note

• BR now support backing up user tables created in the mysql schema.

[YuJuncen]

need #1025

[3pointer]

/lgtm

[kennytm]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• kennytm

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[lonng]

/merge

[ti-chi-bot]

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

Commit hash: 4459e58