Cleanup process_completed_threshold and related state

PtrQueueSet has several related values controlling what action should be taken when a buffer is complete. Some of these values have complicated semantics, with negative or 0 values being special and disabling the normal range-based behavior. Things could be simplified by just using size_t with obvious range-based behavior. The variables in question are:

_process_completed_threshold
_max_completed_queue
_completed_queue_padding