demux: only query stream size at most once per second

Instead of every packet. Doing it every packet led to the performance
regression mentioned in the fstat() commit. This should now be over, but
out of being careful, don't query the file size that often. This is only
used for user interface things, so this should not cause any problems.

For the sake of leaving the code compact, abuse another thing that is
updated only every second (speed statistics).

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