Fix an issue when joining due to inheritance

The delegation/join process already works ok even if a set of joins contains
a subset of another set of joins.  UR::Object::Join->resolve_chain() accepts
a join_label that it tacks onto each Join's ID.  This makes a join chain
    A -> B -> C -> D
have unique joins from
    A -> E -> C -> D
and let the system apply separate SQL filters on D for one chain or the other.

When QueryPlan was adding joins due to inheritance, it wasn't adding on the
join_label (property_name) like UR::Object::Join did.

Change-Id: Id2c0cd24ce2458ce1616ae1d7a12c7171229a1b3