StreamEdge getOrCreateStreamEdge(StreamSpec streamSpec) {
    return getOrCreateStreamEdge(streamSpec, false);
  }
Get the {@link StreamEdge} for a {@link StreamSpec}. Create one if it does not exist.
@param streamSpec spec of the StreamEdge
@return stream edge
samza-core/src/main/java/org/apache/samza/execution/JobGraph.java