A mixture of a multi-byte *TEXT column and a short binary column
produced a too large column.
For example, COALESCE(tinytext_utf8mb4, short_varbinary)
produced a BLOB column instead of an expected TINYBLOB.

- Adding a virtual method Type_all_attributes::character_octet_length(),
  returning max_length by default.
- Overriding Item_field::character_octet_length() to extract
  the octet length from the underlying Field.
- Overriding Item_ref::character_octet_length() to extract
  the octet length from the references Item (e.g. as VIEW fields).
- Fixing Type_numeric_attributes::find_max_octet_length() to
  take the octet length using the new method character_octet_length()
  instead of accessing max_length directly.