DEV-40528 Viewer 2 floods server with AvatarPropertiesRequest messages causing...

DEV-40528 Viewer 2 floods server with AvatarPropertiesRequest messages causing database load.  Icon cache was only being used to provide images more quickly, not to suppress network traffic.  Made it suppress network traffic, which may result in stale icons occasionally.  Converted online/offline tooltip to use LLAvatarTracker.  Will port to viewer-2-qa-1.  Reviewed with Richard.