Lock areas of AsyncCommandManager where multiple threads could try to access/update the same static structures simultaneously.

This is possible where there is more than one scene (multiple copies of the same script engine) and/or more than one script engine being used.
These operations are not thread safe and could be leading to the exceptions/problems seen in http://opensimulator.org/mantis/view.php?id=6651
This also prevents a small race condition where more than one AsyncLSLCmdHandlerThread could be started.