Add support for RCv3 to add_to_load_balancer

This is the first big one, but only in the sense that it stands on the shoulders of giants. Or, more accurately, a tower made of many, many tiny lego bricks :)
FYI, this is because the actual HTTP body response has a list-of-dicts in it (because we're doing bulk requests), but `add_to_rcv3` gets rid of that wrapping list, because it only ever does one LB-server pair at a time.