improve RDY handling

this is a change pulled out of #28 

The code managing that over-subscribed state with self.total_ready is deficient in two ways. 

 1) It doesn't account for multiple tasks (we've implemented in_flight_count as per-task), and
 2) it is never incremented (oops!) when RDY is updated, so it can't be checked against for calculation. It appears that it only works at initial connection time now.