demux: adaptative: rewrite using synchronous demuxers

Can now flush buffers on demux restart.
Do align pcr after sending to decoders instead of always incrementing
by the target value (avoid dropping blocks across segments on restart).
Always issue a fakees to demuxer, then recycle on execution.
Avoids double deletion with duplicate Del commands (demuxer 0..n+self n).
Can now handle HLS discontinuities.
Drops the streamoutput layer.