Keep the HLS backlog even if the stream header changes.

Keep the HLS backlog even if the stream header changes.

The typical case here is if we are streaming fMP4 and the encoder
has to restart. The regular (non-HLS) stream has to cut off the
backlog here, but HLS supports multiple headers, so we can add a
discontinuity to keep the backlog seekable.

I haven't actually found a client that will play correctly across
the discontinuity, but hls.js can at least _seek_ across it,
which is already useful.