add node level timing info to run_results.json and to tracking

[cmcarthur]

Adds node level timing info to RunModelResult. this then propagates to run_results.json and to event tracking.

@drewbanin I think I need to update an IGLU schema for this to work properly, no?

[drewbanin]

@cmcarthur this is really great! I love the context manager -- good idea. I think this is good to ship as-is, with a couple of other notes:

• i like that we can extend this to include pre- and post- hooks at some point in the future. Unsure how we would do that at this very moment, but they'll fit right in when we get there.
• this doesn't include on-run-start/end hooks, which i think is ok b/c they aren't represented in run_results.json at present. If that's an easy thing to do, I think we should do it! If it ends up requiring a ton of refactoring to support, we can punt on it until we generally revisit those hooks.
• I think you're right that we'll need to update an iglu schema here. These things are versioned, and I think we might want to bump the run_model_spec to use 1-0-1. We'll need to make a PR in the relevant repo -- i can get that sorted