libhb: make muxer, sync, and reader behave like other work objects

libhb: make muxer, sync, and reader behave like other work objects

simplify job initialization sequence, clean up code, and document
dependencies in the sequence better.

Make hb_add set job->sequence_id.  It is no longer necessary for the
frontend to do this.  If the frontend needs the sequence_id, it is
returned by hb_add().

Clean up use of interjob.  do_job() now uses sequence_id to detect when
a new sequence of related jobs is running and automatically clears
interjob.