TaskHubWorker to implement IHostedService

This would make it easier for .NET developers to host `TaskHubWorker` instances in [.NET background worker](https://docs.microsoft.com/aspnet/core/fundamentals/host/hosted-services#ihostedservice-interface) projects. It also more closely aligns DTFx with modern .NET design patterns.

In terms of implementation, this shouldn't be very difficult. It's mostly a matter of adding `StartAsync` and `StopAsync` overloads that take a `CancellationToken` parameter. As part of this, we could consider passing down these cancellation tokens to inner StartAsync and StopAsync calls.