Split create_schema_table() into virtual methods in Type_handler

The code in create_schema_table() has switch's and if's testing MYSQL_TYPE_XXX.
This is not friendly to data type plugins.

Additionally, when schema tables are created in two steps:

• On the first step, Items are created from ST_FIELD_INFO
• On the second step, these Items create Fields
  which is slow.

Let's do the following:

• Add a new method in Type_handler:

  virtual Field *make_schema_field(TABLE *table,

  const Record_addr &addr,

  const ST_FIELD_INFO &def,

  bool show_field) const

• Change the logic to create Fields directly from ST_FIELD_INFO.
• For that purpose, and to be able to reuse the code in a convenient way, wrap create_tmp_table() into a class Create_tmp_table with approximately these methods:

class Create_tmp_table

{

public:

  ...

  // The former beginning of create_tmp_table()

  TABLE *start(THD *thd,

               TMP_TABLE_PARAM *param,

               const LEX_CSTRING *table_alias);

  // The former loop of create_tmp_table(), adding fields into the table

  bool add_fields(THD *thd, TABLE *table,

                  TMP_TABLE_PARAM *param, List<Item> &fields);

  // A new code adding fields directly from ST_FIELD_INFO

  bool add_schema_fields(THD *thd, TABLE *table,

                         TMP_TABLE_PARAM *param,

                         const ST_SCHEMA_TABLE &schema_table,

                         const MY_BITMAP &bitmap);

  // The forder end of create_tmp_table(), after the loop adding fields.

  bool finalize(THD *thd, TABLE *table, TMP_TABLE_PARAM *param,

                bool do_not_open, bool keep_row_order);

};

• Remove method Item::create_field_for_schema()
• Remove class Item_return_date_time
• Remove class Item_blob