Added SwitchCounters.java. Similar to SwitchManagerCounters.java, the SwitchCounters class contained within contains and maintains all the counters for a switch instance. When the switch disconnects, either accidentally or forcefully with disconnect(), the SwitchCounters will be removed from the IDebugCounterService, since they will no longer be valid or meaningful if the switch never reconnects.
Changed methodology after realizing the counters are already defined in OFConnectionCounters, which makes more sense since switches can have multiple connections now. OFConnectionCounters now refactored to remove counters when a switch disconnects.