The intention of this stream API is give server application about
stream dependency information, so that it can utilize it for better
scheduling of stream processing.  We have no plan to add object
oriented API based on stream object.

We now use priority queue per stream, which contains the stream which
has ready to send a frame, or one of its descendants have a frame to
send.  We maintain invariant that if a stream is queued, then its
ancestors are also queued (except for root).  When we re-schedule
stream after transmission, we re-schedule all ancestors, so that
streams on the other path can get a chance to send.  This is basically
the same mechanism h2o project uses, but there are differences in the
details.

Previously, the number of stream in one dependency tree (not including
root) is limited to 120.  This is due to the fact that we use
recursive calls to traverse trees.  Now we replaced recursive calls
with loop, we can remove this limitation.  Also now all streams are
descendant of root stream, rather than linked list of individual
subtree root.