Add support for forced prebuffering.

The motivation is jwPlayer, which for HTTP files expects to be able
to do no prebuffering and just download full speed nevertheless
(as it assumes they are static files, not streams) -- when it cannot,
it shows an ugly icon on top of the stream all the time. So we add
an option for forced prebuffering (three seconds seems to be about
fine) which means we wait sending until we have a pretty big backlog.

Ideally, we would be able to actually send old data instead of just
waiting (which would mean that the client doesn't need the extra wait
at the beginning), but it's complicated with having to remember old
keyframe positions, changed stream headers etc.