Draft: Use atomic stack for async job queue

Previously, the async job queue would use a locked-list (ISC_LIST). With introduction of atomic stack (that has to be drained at once), we could use it to remove some contention between the threads and simplify the async queue.

Fortunately, the reverse order still works for us - instead of append and tail/prev operation on the list, we are now using prepend and head/next operation on the atomic stack.