IO/Ops: assign IO::PropState::original in normal store/load instead of in Object::Unit (de)serialize().¹

1: Weak store/load does not actually affect the datastore, so the prop state
should not be changed when weak storing/loading.
Hord::Hive::Unit: renamed has_child() to has_object().¹

Cmd::Node::Create: tidy.

1: Children collection is quite different from the object collection.
Data::Metadata: tidy.
doc: removed redundant parameter descriptions.