config: always skip the system databases

[kennytm]

What problem does this PR solve?

Avoid importing the built-in databases (mysql, information_schema, performance_schema and sys).

What is changed and how it works?

Add these 4 databases into the black list in Adjust().

Check List

Tests

• Integration test

Side effects

• Breaking backward compatibility
  • User has no way to explicitly restore the tables defined inside mysql.

Related changes

• Need to update the documentation
• Need to be included in the release note

[kennytm]

/run-all-tests

[lance6716]

LGTM

and I think user can restore those tables (though not the default way) because

If a database’s name matches both the do-dbs and ignore-dbs arrays, the database is included.

[kennytm]

PTAL @csuzhangxc

[csuzhangxc]

LGTM

[ericsyh]

@kennytm Pls pick to the 3.0 branch

[kennytm]

It's already in the 3.0 branch.