Prepare the controller to accept fallback config cookies instead of host:port strings.

This is necessary for compatibility with the client changes that enable the controller to update its fallbacks anytime.

A fallback config cookie is whatever the controller needs in order to determine whether this client needs its fallback proxies refreshed.  For the time being it just provides the same data as the old host:port scheme.

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 this 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.