Atmos leak fix

This attempts to fix #765 
Hi guys, I added a quick fix which checks for dead entries in a Map before it adds any more.  I ran this for a few days and it seems to works, heap dumps show we are no longer growing this structure.  I was planning on adding a executor or something along those lines to do this at a scheduled interval, but this has been working so well I'm not sure its worth the extra complication (starting, stopping and cleaning up something).  A listener for when these resources expire ( like a session listener in a servlet container) would be nice, but I could not find an obvious way to do that reliably. 

I do want to say this is not something usually I'm comfortable doing, I feel like the little Dutch boy sticking his fingers in the dyke trying to stop each leak!  Unfortunately the atmosphere framework has beaten me ;-), at least how its being used now.  But since we know its not a long term solution for openHAB, I'll keep poking away.