Create a TaskService

[ctrueden]

The TaskService will be an improved version of the StatusService: task-oriented progress tracking, where a task can be made up of subtasks. You can ask the TaskService for its list of pending Tasks, ask for an aggregate progress estimate, etc.

[ctrueden]

We can also add a "time remaining" estimate feature!

[ctrueden]

As of b484990, the TaskService exists. We do not yet have a Task implementation that consists of subtasks, nor has the TaskService fully subsumed the StatusService yet, so I'm going to wait to close this until the service is further developed.