GENIUS-237 hit an issue of apparent excessive toString(), and turns out to be due to be just because JobCoordinator's enqueueJob() requires a String key argument.

Looking into this see, we see that JobCoordinator's implementation doesn't actually really require a String - just a key for a Map (so that "Jobs with the same key are run sequentially. Jobs with different keys are run in parallel.")

Why don't we therefore change the JobCoordinator API & Impl so key can be any Object instead String - that way we can save caller any "artificial" (unnecessary) toString invocations.