Optimize dependency based priority code to Firefox style tree

While this commit optimizes dependency routine to Firefox style tree,
the other use cases (e.g., linear chain) are also improved
dramatically as well.

lib/nghttp2_session.c

@@ -6306,8 +6306,8 @@ int nghttp2_session_pack_data(nghttp2_session *session, nghttp2_bufs *bufs,
     return rv;
   }
 
-  session_outbound_item_schedule(session, stream->item,
-                                 stream->effective_weight);
+  session_outbound_item_schedule(
+      session, stream->item, nghttp2_stream_compute_effective_weight(stream));
 
   return 0;
 }

lib/nghttp2_stream.h

@@ -209,13 +209,14 @@ struct nghttp2_stream {
   int32_t local_window_size;
   /* weight of this stream */
   int32_t weight;
-  /* effective weight of this stream in belonging dependency tree */
-  int32_t effective_weight;
-  /* sum of weight (not effective_weight) of direct descendants */
+  /* sum of weight of direct descendants */
   int32_t sum_dep_weight;
   /* sum of weight of direct descendants which have at least one
      descendant with dpri == NGHTTP2_STREAM_DPRI_TOP.  We use this
-     value to calculate effective weight. */
+     value to calculate effective weight.  This value is only
+     meaningful iff dpri == NGHTTP2_STREAM_DPRI_NO_ITEM and all
+     streams along the path to the root stream (follow dep_prev) have
+     NGHTTP2_STREAM_DPRI_NO_ITEM. */
   int32_t sum_norest_weight;
   nghttp2_stream_state state;
   /* status code from remote server */
@@ -325,21 +326,12 @@ int nghttp2_stream_dep_subtree_find(nghttp2_stream *stream,
 
 /*
  * Computes distributed weight of a stream of the |weight| under the
- * |stream| if |stream| is removed from a dependency tree.  The result
- * is computed using stream->weight rather than
- * stream->effective_weight.
+ * |stream| if |stream| is removed from a dependency tree.
  */
 int32_t nghttp2_stream_dep_distributed_weight(nghttp2_stream *stream,
                                               int32_t weight);
 
-/*
- * Computes effective weight of a stream of the |weight| under the
- * |stream|.  The result is computed using stream->effective_weight
- * rather than stream->weight.  This function is used to determine
- * weight in dependency tree.
- */
-int32_t nghttp2_stream_dep_distributed_effective_weight(nghttp2_stream *stream,
-                                                        int32_t weight);
+int32_t nghttp2_stream_compute_effective_weight(nghttp2_stream *stream);
 
 /*
  * Makes the |stream| depend on the |dep_stream|.  This dependency is