Thanks for reviewing @myleshorton!

We tried the ProxyTracker interface first but IIRC we hit some snag and figured we'd punt and come back to it later. Now that it's working with DefaultProxyTracker I'll look at this again.

As for storing checkFallbacksMode, the model was just a convenient place to store it for the places we wanted to check it within the same run (e.g. [here](https://github.com/getlantern/lantern/pull/1481/files#diff-80e1bb7d9fcf8f06fa9c588e75713326R77)), definitely doesn't need to be saved across runs. Probably isn't even really necessary in the ModelIO constructor, was just trying to minimize S3 config checking. Will take it out.

>  I'll also fix the things I broke with the config fetcher stuff -- I can just merge from master...

All good, I just tried merging from master into this branch and got conflicts in Launcher.java, which I resolved easily, and S3ConfigFetcher which I might have to look more at with Snyder tomorrow. Keep you posted!