Replace Virtual_column_info::field_type to Type_handler

Column_definition::check(THD *thd) has the following code:

   if (vcol_info)

   {

     DBUG_ASSERT(vcol_info->expr);

     vcol_info->set_field_type(real_field_type())

To be pluggable data type friendly, we'll need to remove all calls for Column_definition::real_field_type() eventually.
To get rid of this entry, let's do the following:

• Remove the member Virtual_column_info::field_type
• Derive Virtual_column_info from Type_handler_hybrid_field_type
• Remove methods set_field_type() and get_real_type() and use Type_handler_hybrid_field_type::set_handler() and Type_handler_hybrid_field_type::typ_handler() instead.