Add elapsed time to digraph edges

Created by: davecromberge

The time on each edge should be the cumulative time of the children or descendent edges. Therefore the timer on a parent is started before that of its children.

My previous implementation used timing annotations only on debug_flows, thus allowing timers to be inserted as probes into certain areas of the dflow. However, I believe the approach in this PR to be superior, as the graph is less cluttered.