The structure of THD is not available inside plugins. So a plugin cannot do:

  new (table->in_use->mem_root) Field_xxx(...)

Let's add a MEM_ROOT pointer parameter to these Type_handler methods:

• make_conversion_table_field
• make_table_field
• make_and_init_table_field
• make_schema_field

Plugins will create fields as follows:

  new (mem_rroot_arg) Field_xxx(...)

Note, we eventually should probably change the TABLE parameter to to make_xxx_field() to a subset of TABLE which is only necessary inside a plugin.