parallel gateway bug fixes

allow refs to vars from any branch of a parallel gateway.

fix bug where gateway validation was being short circuited due to cyclic visit check. we now maintain a map of "visited" elements (enabling cycle detection) per branch of a gateway rather than a global basis. What was previously happening was that a convergent gateway was being detected as a "cycle" when it isn't, it's just that it had been visited on traversal of a prior branch of a gateway.