- Adding new virtual methods in Type_handler:
  * Column_definition_prepare_stage1()
  * Column_definition_prepare_stage2()
  * calc_pack_length()

- Using new methods to remove type specific code in:
  * Global function calc_pack_length()
  * Column_definition::prepare_create_field()
  * The loop body mysql_prepare_create_table()
  * Column_definition::sp_prepare_create_field()