This patch solves two key problems.
1. There is a type number clash between MySQL and MariaDB. The number
   245, used for MariaDB Virtual Fields is the same as MySQL's JSON.
   This leads to corrupt FRM errors if unhandled. The code properly
   checks frm table version number and if it matches 5.7+ (until 10.0+)
   it will assume it is dealing with a MySQL table with the JSON
   datatype.
2. MySQL JSON datatype uses a proprietary format to pack JSON data. The
   patch introduces a datatype plugin which parses the format and convers
   it to its string representation.

The intended conversion path is to only use the JSON datatype within
ALTER TABLE <table> FORCE, to force a table recreate. This happens
during mysql_upgrade or via a direct ALTER TABLE <table> FORCE.