Revert "Revert "MSDN documentation is unclear about whether exiting a lock() block will trigger a Monitor.Wait() to exit, so avoid some locks that don't actually affect the state of the internal queues in the BlockingQueue class.""

This reverts commit 21a09ad3ad42b24bce4fc04c6bcd6f7d9a80af08.

After more analysis and discussion, it is apparant that the Count(), Contains() and GetQueueArray() cannot be made thread-safe anyway without external locking
And this change appears to have a positive impact on performance.
I still believe that Monitor.Exit() will not release any thread for Monitor.Wait(), as per http://msdn.microsoft.com/en-gb/library/vstudio/system.threading.monitor.exit%28v=vs.100%29.aspx
so this should in theory make no difference, though mono implementation issues could possibly be coming into play.