in src/main/java/com/spotify/futures/ConcurrencyReducer.java [136:147]
private void pump() {
Job<T> job;
while ((job = grabJob()) != null) {
final CompletableFuture<T> response = job.response;
if (response.isCancelled()) {
limit.release();
} else {
invoke(response, job.callable);
}
}
}