Refactor TTL esp in regards to templated scenes

[hi-liang]

Splitting this one one off

Deal with TTL exception for templateScene, whereby expireAt is not set in the mongo document (what actually enforces expiry behavior in mongodb). Perhaps via flags where persist: true, ttl: n are set, but some new flag (retainTTL: true ?) does same bypass of expireAt so every time someone loads the scene from persist (or instantiates a sceneTemplate), the object is kept with a "fresh" TTL

Originally posted by @hiliang-cmu in #4 (comment)

[hi-liang]

Even worse wiith the ugly workaround to keep a Set of expirations, this should be redone to better handle time in an absolute manner, something previously avoided because of the lack of time sync.