Fix oj errors

See #656.

Relaxes `oj`'s dump mode to `:compat`, centralizes all JSON (de-)serialization, uses different compatibility modes for load and dump (following the example of `multi_json`'s `oj` adapter) and removes any explicit `.to_json` calls (and `mimic_json` from `oj`) for the avoidance of doubt.
I'm steering clear of the actual method_name `to_json`, due to it being a (conflicted) playground for too many gems. Cleaner to namespace our own method and use that everywhere -- means we can swap out implementations, or configure them, in that file alone.