Implement stats.loadAsyncGraph

[aykutalparslan]

Load channel statistics graph asynchronously

statsGraphAsync#4a27eb2d token:string = StatsGraph; statsGraphError#bedc9822 error:string = StatsGraph; statsGraph#8ea464b6 flags:# json:DataJSON zoom_token:flags.0?string = StatsGraph; ---functions--- stats.loadAsyncGraph#621d5fa0 flags:# token:string x:flags.0?long = StatsGraph;