Lazy Elapsed filter

For some metrics gathering I needed a filter like the Elapsed filter, but I needed it to be lazy about how it picked the ending event since in my situation I would receive multiple end events that may come in any order or might not come at all. I also needed to record pieces of each event seen and also tally up a numerical field from each event. So, I created the Lazy Elapsed filter to solve my problems and I figured it might solve someone else's problems as well. 

This pull request adds the new filter and it's associated tests. 

Thank you for your consideration and comments.