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.
Obviously non-blocking, but I had a similar problem when writing the optimizer, where this line before partition_groups just became way too long. If it starts bothering you, I had good results with groupby (i.e. just let it produce a dict, and then you can do something like servers[ACTIVE], servers[DRAINING] etc).