[cooja] skins/TrafficVisualizerSkin: Added @Override annotations and added some final modifiers
[cooja] skins/TrafficVisualizerSkin: Indicate sent but unreceived messages by a red double circle around mote
[cooja] skins/TrafficVisualizerSkin: Use LinkedList instead of ArrayList to increase add and remove performance
[cooja] skins/TrafficVisualizerSkin: Removed unnecessary 'historyCopy'
[cooja] skins/TrafficVisualizerSkin: Reduce array copy and iteration overhead
[cooja] skins/TrafficVisualizerSkin: Age handling moved to RadioConnectionArrow class which also got some lines of function documentation
[cooja] skins/TrafficVisualizerSkin: Use alpha value to fade out arrow color an potentially allow to configure colors
[cooja] plugins/sinks/TrafficVisualizerSkin: Synchronized to avoid ConcurrentModificationExcpetions
Merge pull request #769 from ejoerns/pull-req/cooja-traffic-vis

[Cooja] TrafficVisualizerSkin modifications