Make sure MsmqWorkerAvailabilityManager is thread safe on .NET 4 in NSB 3.3.9

MsmqWorkerAvailabilityManager.ClearAvailabilityForWorker() throws "System.InvalidOperationException: Property ResponseQueue was not retrieved when receiving the message. Ensure that the PropertyFilter is set correctly" if you put some stress on it with NSB 3.3.9 on .NET 4+.

The problem is that Microsoft made almost everything on MessageQueue NOT thread safe between .NET 3.5 and .NET 4. 

3.5 docu: http://msdn.microsoft.com/en-us/library/system.messaging.messagequeue(v=vs.90).aspx vs
4 docu: http://msdn.microsoft.com/en-us/library/system.messaging.messagequeue(v=vs.100).aspx

Those changes by Microsoft makes the MsmqWorkerAvailabilityManager implementation in 3.3.9 not safe, as it needs locking to work. The proposed fix, which fixes the repro, is basically to revert  @7de28f8f1dbfdf39a810800b841b0df49f02aac6. Bad luck on that one, @andreasohlund. :)

Check out repro, available [here](https://github.com/janovesk/nsb-msmqdotnet4bug).

Well.. You could get a crazy race condition here if somebody calls ClearAvailabilityForWorker(), PopAvailableWorker() or WorkerAvailable() on another thread between the new MessageQueue and the Transactional check.. Should be ok to removed the lock as long as we know Start will always be called (and finish!) before anything else uses MsmqWorkerAvailabilityManager.