Add a '$singleton' option for graph points to keep only the latest single record

Many times a graph point is intended to only have one record stored, never multiple. If we added a '$singleton' boolean option it would allow the dev to specify that an object in the graph was to only have one record, the latest one, kept and all others discarded. Thing of the case of having a blog protocol where you have an `index` html record where you only ever want one, don't really care about the recordId, and just want the latest kept. The $singleton option would make that possible.