Function read_into 

pub fn read_into<R: Read>(
    open: impl FnOnce() -> Opened<R> + Send + 'static,
    stop: impl Fn() -> bool,
    write: impl FnMut(&[u8]) -> Result<()>,
) -> Result<u64, StreamError>

Run open on its own thread and pass each chunk to write here, in order, returning the bytes written (a stated length must match). Reads run at most QUEUED_CHUNKS ahead; a silent server holds the reader, not this thread, with stop checked between chunks and every STALL_CHECK. The reader ends at its next chunk once this side goes.