When metrics system is restarted MBean names get incorrectly flagged as dupes

In the Metrics2 system, every source gets registered as an MBean name, which gets put into a unique name pool in the singleton DefaultMetricsSystem object. The problem is that when the metrics system is shutdown (which unregisters the MBeans) this unique name pool is left as is, so if the metrics system is started again every attempt to register the same MBean names fails (exception is eaten and a warning is logged).
I think the fix here is to remove the name from the unique name pool if an MBean is unregistered, since it's OK at this point to add it again.