Added option to remove debug counters from the hierarchy if it's been ad...

@capveg Added option to remove debug counters from the hierarchy if it's been added; useful for keeping track of switch or device counters, which might not stick around forever. Fixed bug in CounterNode where non-null was returned regardless of whether a counter was already present or was a new counter. DebugCounterServiceImpl prints a debug message if non-null is returned, mistakenly indicating you're adding a counter that you already had registerd (generated TONS of annoying incorrect debug messages at startup). Next bug... I accidentally initialized counters twice in the OFSwitchManager; now only done once.