Merge MySQL 5.7.21

Registered by Laurynas Biveinis

MIGRATED TO https://jira.percona.com/browse/PS-307

Merge MySQL 5.7.21.

Verify upstream fixes and revert ours, keeping the testcases as needed:
- https://bugs.mysql.com/bug.php?id=85371 / bug 1667307
- https://bugs.mysql.com/bug.php?id=86209 / bug 1688698

Blueprint information

Status:
Not started
Approver:
None
Priority:
Not
Drafter:
Laurynas Biveinis
Direction:
Approved
Assignee:
None
Definition:
New
Series goal:
Accepted for 5.7
Implementation:
Deferred
Milestone target:
None

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