DAG of Flowlets instead of Tree of Flowlets

[reshadi]

should we support flowlets as a DAG instead of tree.

There might be cases that a flowlet is the join point of multiple other flowlets. For example, if we have batching mechanism that "merges" multiple flows into one, we might want to represent a new flowlet for the batched activity that summarizes the incoming flowlets.

We can potentially support this model by adding an optional readonly parents?: Flowlet[] to the Flowlet class. That way, we can handle the flow as a DAG instead of a tree.