Healthcheck and status updates during TurbiniaTask.exec()

[aarontp]

The exec() method is currently synchronous, but it would be nice if it would run things in the background and give status updates while long-running tasks are still running. We could also add health-checks which could look for things like whether a give file output is changing size or MAC times, and kill the task if the health checks fail. Maybe each task could have an optional Task.healthcheck() method.