Merge MySQL 5.6.27

https://github.com/percona/percona-server/pull/211

Merge MySQL 5.6.27.

Verify upstream fix and revert ours:
- https://bugs.launchpad.net/percona-server/+bug/1380985 / https://bugs.mysql.com/bug.php?id=76349
- https://bugs.launchpad.net/percona-server/+bug/1308016 / http://bugs.mysql.com/bug.php?id=76927

Check https://bugs.mysql.com/77667, add testcase if fixed.

Verify upstream fix for https://bugs.mysql.com/76727, re-enable our testcase if OK, re-enable savepoints in triggers and stored functions.

Add a testcase for 74589 (missed in 5.6.24)
Add a testcase for 74778 (missed in 5.6.25)
Add a testcase for 74851 (missed in 5.6.23)
Add a testcase for 74950
Add a testcase for 75230 (missed in 5.6.24)
Add a testcase for 76946

Fix for http://bugs.mysql.com/bug.php?id=72811 should be reconciled with our NUMA feature