Look into the initPlans attached to lefttree of RemoteSubplan while deciding

Look into the initPlans attached to lefttree of RemoteSubplan while deciding
whether correct variables are being referenced

This helps us to fix issue #81. It's not immediately clear if we should handle
this is more elegant manner than what we've done here. In PostgreSQL the
initplans are always attached to the top level plan, but in XL we add a
RemoteSubplan node on top of the top level plan. Unless we take into account
vars generated by the initPlans, we might incorrectly conclude that certain var
is not accessible by the subquery.