Fix select concurrent

### Story

https://www.pivotaltracker.com/story/show/80642438

### Notes

* When selecting concurrent events, we now use the same criteria used for sorting to determine whether events are concurrent for the purpose of FIRST, etc
* The equals operators for TS and IVL_TS (which may in fact no longer be used with the changes to selecting concurrent events) didn't correctly handle null dates, and had some other issues as well