Fixes for proxy-mode pool switching

Requires PR from node-stratum-pool project as well, so don't merge one without the other.

Switching works well and I added some docs to the example configuration file to help illustrate.  I still need to follow up with restoring the current proxy pool from redis so the state will persist after a reload/restart.

I'd also like to see the difficulty carry over to the new client when the switch happens - there is more work to make that happen as well.