Move save() / delete() logic in Field / FieldInstance to [pre|post]Save(), [pre|post]Delete()

Those have just been added by #1893772: Change notice: Move entity-type specific storage logic into entity classes.

Entity::save() & Entity::delete() should be left as the parent implementations that just delegate to the storage controller. The storage controller then calls [pre|post]Save(), [pre|post]Delete(), etc...