Allow the controller to reset the client's list of fallback proxies

The process to deploy this should be:

- Merge and deploy the feed-fallbacks branch in the controller.
- Pause proxy creation in the cloudmaster (e.g. by renaming /home/lantern/cloudmaster.py),
- Merge the new client version, build and release installers,
- Merge the feed-fallbacks branch in lantern_aws, Rebuild wrappers. Upgrading Lantern in the proxies themselves may be a good idea, but it's not strictly necessary. The controller handles old clients alright.