Healthcheck and status updates during TurbiniaTask.exec()

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.

Start by reading the TurbiniaTask.exec() entry point and the Task interface, then determine how long-running tasks currently report completion. Define how background execution, status updates, optional Task.healthcheck() methods, output-file size or MAC-time checks, and termination on failed checks should fit together. Done should include an agreed design and coverage for the new behavior.

Written by the indexing model from the issue text.