Method StopBackgroundCleanUpLoop in class EtpProtocolHandler not thread safe

[waveflight]

In testing, encountered exception: Object reference not set to an instance of an object on line 1025 of the EtpProcolHandler class. Note that the check on line 1024 is not thread safe.

if (_backgroundCleanUpTask != null)
                    **AsyncContext.Run(() => _backgroundCleanUpTask.ConfigureAwait(Session.CaptureAsyncContext));**

There are 2 separate locations that the StopBackgroundCleanUpLoop method is called from. I did not investigate under what circumstances both will execute, but I suspect that this is how this exception is encountered. The simple fix for now is to make the StopBackgroundCleanUpLoop thread safe.