Improve/Enhance Is(Self)Busy to handle nested busy scenarios better

[StefanOssendorf]

Currently Is(Self)Busy is only a bool. This must not change with respect to the public api.
But handling that bool should be instead an integer to count busyness and non busyness.
Something like the following would today end the "busyness" early eventhogh it's not expected/desired:

public void Foo()
{
  try
  {
    MarkAsBusy();
    // more locs
    Bar();
    // more locs
  }
  finally
 {
    MarkAsIdle();
 }
}

public void Bar()
{
  // Do stuff here
  MarkAsIdle();
}

With such a structure the busyness would end early. With an internal counter we could more easily handle the busy flag.
Something like this:

private int _isBusyCounter;

MarkAsBusy()
{
  Interlocked.Increment(ref _isBusyCounter);
  // Other stuff the implementation needs
}

MarkAsIdle()
{
  Interlocked.Decrement(ref _isBusyCounter);
  // Other stuff the implementation needs
}

With this as long as the dev ensure he's calling MarkBusy() and MarkIdle() in pairs the busy state will always be correct without worrying about where the method is used.