nodestack: revert may_read_from to a normal member variable, not pointer

nodestack: revert may_read_from to a normal member variable, not pointer

Because we don't control the 'new' operator for STL sets, I can't properly
ensure that it is allocated in non-snapshotting memory. So just make
may_read_from a normal member variable within class Node, so that its
initial 'allocation' is performed as part of the instantiation of its
subsuming Node object.