Redis migration

This is the code for the redis migration backend.

It takes an old and a new backend, and tries to copy existing keys to the new database before answering anything.
I found the matter.
It cames from https://github.com/mozilla-services/loop-server/blob/master/loop/websockets.js#L197-L201

What happens is that we send the hello init back before having finishing the storage calls. We should fix that.