gom: Ensure we update ids when saving a resource group

We move the freeing of the items to the destruction of the asyncresult
and then we iterate over all items calling gom_resource_set_post_save_properties
on a successfull operation.

This also updates test-gom-stress to ensure we're getting the right ids
when calling gom_resource_group_write_sync().

https://bugzilla.gnome.org/show_bug.cgi?id=747849