It turns out that read preference, when set on a DBCollection, applies
across the board, since DBCollections are shared (see
com.mongodb.DBApiLayer.doGetCollection()).

Instead we must set the read preference on each find() within
MongoLookupEntryStore. More error prone, but I don't see another way
around the shared DBCollection objects.
Merge pull request #107 from atlasapi/only-content-writer-reads-from-primary-for-lookup-queries

Only do lookup queries on the primary when writing content, not during other API actions