Use correct stream for Delayed Reading

This pull request ensures that a delayed read of a product by the ROOT/EDM input source (PoolSource) reads the correct entry in the Events tree in the multithreaded framework, where events are processed in parallel.  This requires a mapping between the stream ID of the event to the entry number in the events tree.  Without this pull request, a delayed read of a product would likely read it from the entry in the Events tree corresponding to the most recent event read, rather than the correct event.