Probably not noticed until now because the queue is large enough that two
threads each filling theirs at the same time and flushing is unlikely to
happen.
Also made explicit that each worker thread gets its own queue.
I think that was the case before, but if something was put in the queue
before worker threads were forked off, they could have each inherited the
same queue.
Could have gone with a single shared queue, but per-worker queues is more
efficient, because a worker can add lots of stuff to its own queue without
any locking.
This commit was sponsored by Ole-Morten Duesund on Patreon.