Avoid calling `result()` in task callbacks

[gpauloski]

Each task created by the Engine has a callback which will call result() on the future to get the TaskInfo for logging. This has the unfortunate side-effect of pulling the actual task result onto the client, which could incur some non-trivial data movement cost depending on the executor and app.

We might be able to avoid this by changing the TaskInfo to get passed on to child tasks, and only logged if TaskFuture.result() is called. This means that:

• A child task failing could mean we lose the TaskInfo of parent tasks.
• A parent with multiple children could each log the parent's TaskInfo so we'd need to check if task IDs were already logged.
• If the app never calls result(), the TaskInfo for possibly many tasks in a chain do not get logged.

I think we can solve all of these.