fast navigation scroll

PR to fix #373 issue.
Users can quickly swipe to some very past period without having to wait. The reports http request is cancelled for periods which data didn't manage to load before. The app is stable tapping many times.
future improvements: object pool and memory profiling.

HttpRequestException shouldn't be logged as an error.. network errors are common, so only log it at info level.

Also, you should probably use `IsNetworkFailure` extension method like [in SyncManager](https://github.com/toggl/mobile/blob/master/Phoebe/Net/SyncManager.cs#L123-L130).