Merge MySQL 5.7.20

Registered by Laurynas Biveinis

https://github.com/percona/percona-server/pull/1984

Merge MySQL 5.7.20.

Verify upstream fixes and revert ours:
- https://bugs.mysql.com/bug.php?id=81714 / bug 1588845
- https://bugs.mysql.com/bug.php?id=70490 / bug 1205196
- https://bugs.mysql.com/bug.php?id=82980 / bug 1619622
- https://bugs.mysql.com/bug.php?id=87518 (85835) / bug 1679025

Add testcases for:
74898 (internal #20049521)
75721=75741=75707=75234=75854=74583 (internal #19894987, #20449914)
74953 (internal #17846865)
81472 (internal #23303485)
83899=82652 (internal #24489302)
77522=77523=77527 (internal #20857652)

Add '--source include/have_myisam.inc' to 'main.bug81449' and 'main.bug84640' MTR test cases

Blueprint information

Status:
Complete
Approver:
Laurynas Biveinis
Priority:
Essential
Drafter:
Laurynas Biveinis
Direction:
Approved
Assignee:
Yura Sorokin
Definition:
Approved
Series goal:
Accepted for 5.7
Implementation:
Implemented
Milestone target:
milestone icon 5.7.20-18
Started by
Laurynas Biveinis
Completed by
Yura Sorokin

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