store mysql_upgrade version info in system table instead of local file

Currently mysql_upgrade stores version info in file $datadir/mysql_upgrade_info

This has a set of disadvantages:

• information is not accessible from the SQL level
• when restoring a backup from an older server version the file information will not match the actual database version state
• mysql_upgrade can only be executed successfully by an OS user with write access to the datadir
• mysql_upgrade can perfectly perform its upgrade tasks while running remotely on a different host, but will write the version info file locally then

All of these points would be solved if upgrade version information would be stored in a system table instead of a local file ...