Store entities (not UUIDs) in Liberator context

[DaoWen]

We're currently caching the UUIDs of our jobs/instances/groups in the Liberator context when processing the decision handlers for our REST endpoints. That choice results in a lot of repeated UUID→Entity conversions, which probably aren't too expensive, but likely aren't negligible either.

We could eliminate a lot of duplicate computation in the handlers by storing datomic entities (rather than UUIDs) in the context. I'm not sure if this will result in a measurable speedup, but it seems worth exploring. Even if there's no speed improvement, I think the code complexity reduction would be a clear win.