MDEV-10780 Server crashes in in create_tmp_table
MDEV-11265 Access defied when CREATE VIIEW v1 AS SELECT DEFAULT(column) FROM t1

Item_default_value and Item_insert_value erroneously derive from Item_field
but forgot to override some methods that apply only to true fields,
so the server code mixes Item_{default|insert}_value instances with real
table fields (i.e. true Item_field) in some cases.
Overriding a few methods to avoid this.

TODO: we should eventually derive Item_default_value (and Item_insert_value)
directly from Item, as they don't really need the entire Item_field,
Item_ident and Item_result_field functionality.
Only the member "Field *field" related functionality is actually needed,
like val_xxx(), is_null(), get_geometry_type(), charset_for_protocol(), etc.