Reverting @7de28f8f1dbfdf39a810800b841b0df49f02aac6. You need the locking when running on .NET 4+. Everything became thread UNSAFE between .NET 3.5 and 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
Relaxed the locking somewhat. More targeted locking in PopAvailableWorker, removed locking from Start().
Merge branch 'janovesk-DistributorDotNet4Fix' into hotfix-3.3.10
Merge branch 'hotfix-3.3.10' into support-3.3