Implement echo backends

[pgunn]

Add ability to configure multiple backends to serve the same schema, with one main and several echos. An echo backend never has its reponses make its way back up to the client under any circumstances, but its responses may be logged, and mongoe may be configured to compare its responses to the main (although I don't know if this makes sense given that ordering concerns may result in different JSON being returned for the same query - figure this out!)