InnoDB Clone Rollback Signal for Other Engines

The current version of this page is at https://github.com/facebook/mysql-5.6/wiki/InnoDB-Clone-Rollback-Signal-for-Other-Engines

If InnoDB clone decides to rollback during its startup, any other participating storage engines will not be aware of this and will proceed as usual, resulting in an inconsistent instance. Ideally this should be fixed through a 2PC-like design where all the SEs vote to commit the clone, but for the case of two-SE InnoDB/MyRocks clone where InnoDB always initializes first, a simpler solution suffices: make InnoDB write a special marker file if it decides to rollback on cloned instance startup.