With the implementation of the new data dictionary, the metadata will
be stored in the InnoDB tables. For the users to upgrade from the older
MySQL version(5.7) to mysql-8.0, dictionary tables should be created and
populated in old data directory from various metadata sources.

When the Mysql-8.0 server comes up, it detects for the presence of
dictionary tables.  If dictionary tables are not present,
they will be created in old data directory and populated
with the metadata. Then the server will proceed with normal start.

If dictionary tables are present, server will proceed with normal startup.

Upgrade Steps for Users
------------------------

1. Do a pre requisite check on 5.7 data directory.
2. Do a slow shutdown of mysql-5.7 server. Slow shutdown involves setting
   global variable 'innodb_fast_shutdown' to zero before server shutdown.
3. Start mysql-8.0 server on 5.7 data directory.
   mysql-8.0 server will detect if Dictionary tables are present or not.
   If dictionary tables are not present,  they will be created in old data
   directory. Metadata will be populated in dictionary tables.
   Then server will proceed with normal start.
4. Execute mysql_upgrade client tool.
5. Shutdown and start server again with normal configuration. (Recommended)

Reviewed-by: Nisha Gopalakrishnan <nisha.gopalakrishnan@oracle.com>
Reviewed-by: Thayumanavar Sachithanantham <thayumanavar.x.sachithanantha@oracle.com>
Reviewed-by: Dmitry Lenev <dmitry.lenev@oracle.com>