TaskClient should run TaskScheduler in its own thread.

[brthor]

In its current implementation, the TaskClient runs the TaskScheduler in the same loop it uses to execute tasks.

Tasks of any duration more than a second will cause the TaskScheduler to start running behind, scheduled tasks to be moved to the execution queue late, and recurring tasks to be potentially missed.

As a part of fixing this issue, tests should be developed which deliberately use long-running tasks and prove that scheduled tasks are not moved to the execution queue late, and recurring tasks are not missed.