Pull changes to collect domain fronting dialing stats

Closes getlantern/lantern#1913  Includes changes that require changes on the Java side, see https://github.com/getlantern/lantern/pull/1921
This drops the particular update, not the stat.

This is not just a nit on the comment: I think in this case we should ideally mark the stat as invalid altogether to avoid reporting false data (e.g. if we're missing `add`s in an accumulated stat due to load).

I don't think this consideration is worth too much rework at this point, and I'm not sure this can be profitably done locally, but perhaps something to keep in mind for the future.