The queue needs to track each frame/packet's stream index, this is
achieved by maintaining a parallel AVFifo instance for that purpose.
This is simpler than implementing custom AVContainerFifo callbacks.
This ensures that tq_receive() will always return EOF after all streams
were receive-finished, even though the sending side might not have
closed them yet. This may allow the receiver to avoid manually tracking
which streams it has already closed.
