Make derived tables more close internally to views and allow its update.

Derived can be checked and marked for update allowance in mysql_derived_init. All check procedures (like can_be_merged() and view updatability check) now connected to LEX, but easily can be moved to SELECT_LEX_UNIT.

Single table insert/update processed in mysql_derived_merge_for_insert() with help of TABLE_LIST::single_table_updatable().

Then insert/update specific procedures can be changed: check_view_insertability() check_key_in_view()