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.