Converge draining

Implementing the logic for https://github.com/rackerlabs/otter/issues/715

Connections aren't a stat we can get yet, so connections will always be None for now (and so will never == 0).  Just wanted to add this logic so that when we do get the stats it's easy.
Can `current_lb_state` be renamed to `nodes_with_server` or even just `nodes` to emphasize it is list of `LBNode`?