Wanted to use class WorkerThread for DB update, but ran into some of its weaknesses.

Among them is that a thread once it has completed, cannot be re-started without implementation hassle.

While trying to accomplish this, I modified this class a little, but in the end decided to favor the native C# Task class.

So my changes to this class are:
- made access to its WorkState possible from outside (otherwise the thread state cannot be determined)
- added a missing assignment to that state of value "InProgress", which before never occured
- corrected the logic of the IsBusy() function
- but finally: marked this class as obsolete