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.
Hmm... actually this would result in a bunch of other changes due to rewrapping other long lines, so I'm just going to rewrap this line for now.