Easy upgrade from MySQL 8.0 to MariaDB

This is a list of things that needs to do if we would like to provide an 'Easy upgrade' from MySQL 8.0 to MariaDB.

1) Using mysqldump

• This patch should be done in any case, as it should always work independent
  of future changes in MySQL.
• Create a script / program to be run before taking down MySQL
• Start by scanning all MySQL tables and ensure that the tables can be
  moved to MariaDB.
• The script would invoke mysqldump with the optimal options to create
  fastest possible dump & restore, for example using -T (create a tab
  separated file for each table).
• If using -T, enhance mysqldump to be combine -T an --all-databases
• In this case we should store data in 'database/tablename'
• Optionally add a mysqldump option for compressing datafile after
  it was created (in MariaDB we could also add the option to the server).
• Add an option to LOAD DATA to support reading compressed files
  (gzip header could be detected automatically)
• Ensure we can dump system tables in a format usable by MariaDB.

After running the script, there are two options for the end user:

• Take down MySQL and install MariaDB.
• Copy the 'table dump' to another machine with MariaDB installed. This
  could later be put as a slave for the original MySQL version.
• We should create another script to safely read the dumped tables
  into MariaDB. This could add addition checking/work to ensure that
  all system tables are converted properly.

2) In place conversion of MySQL 8.0 to MariaDB

• Take down MySQL (cleanly!) and install MariaDB.
• Start script/program to handle MariaDB to MySQL upgrade
• Some part of the logic, like changing system tables to MariaDB, could
  be done by an external script. To be decided later

Problems to fix:

• Handling MySQL partition tables (same problem as in MySQL 5.7 -> MariaDB) (fixed)
• MySQL new extended timestamp
• Ensure that InnoDB can start with MySQL ib_logfiles (maybe these has to
  be automatically recreated, but should not be a problem if a clean shutdown
  was done).
• Create .frm files for each table from the MYSQL system tables.
• Can be done by using the system tables to generate a CREATE TABLE statement
  (such a program already exists) and use MariaDB internals to create an
  .frm file (most code already exists in MariaDB)
• Loop over all InnoDB tables and convert them to MariaDB format.

The known compatibility issues with InnoDB in MySQL and MariaDB:

• Different log formats (should not be a problem in case of clean shutdown)
• Different InnoDB tablespace / table format (generally)
• Different InnoDB row format when instant alter table was used to add, drop
  or swap columns. There may also be difference when adding/dropping keys.
• If it would be 'close to impossible' to convert these, one option would be
  to run a script on the original MySQL server to do a full table rebuild
  with ALTER TABLE for all tables. This would remove all versioned rows
  from the InnoDB tables.

The above can of course be done on a MySQL slave and after the conversion is
done it should be used to catch up with all data changes before retiring MySQL.