Lock contention in QueueImpl

This is the follow up on https://github.com/hornetq/hornetq/pull/831 .

I couldn't find a way to reopen the old PR, so a new one. 
And I got a bit lost with Git. I'm learning that on the fly here, so I hope everything went fine. Although I'm not 100% sure. 

In any case, I ran the test suite as specified with this change set and they all passed.