Avoid round trips from job-entities -> uuids -> job-entities in generating /list API endpoint.

[scrosby]

In cook.mesos.api/list-jobs, we lookup jobs by entity, then map them to UUID.
Then in https://github.com/twosigma/Cook/blob/master/scheduler/src/cook/mesos/api.clj#L2164, we call cook.mesos.api/fetch-job-map that maps the UUID's back into job-entities. Lets not do this round trip.

[dposada]

Is this a duplicate of issue #617 ?

[DaoWen]

I'm closing this as a duplicate of #617—but I reported #617, and I wholeheartedly agree that we should fix this!

[scrosby]

Its related to #617 in that I'm avoiding round-tripping to and from UUID. But its not a duplicate. Fixing this does not involve any changes to what goes into the liberator context --- the dataflow is via function arguments.