more robust cancelation, notably of passive futures without scheduler running;

eliminated cache, which complicates the code without making a real difference (NB: deque_towards often disrupts caching, and daisy-chaining of workers already reduces queue overhead);

tasks of canceled groups are considered "ready" -- enables to purge the queue from tasks depending on unfinished promises (also improves general reactivity); shutdown: back to synchronous wait, which means no asynchronous interrupts within the loop;