Multi-node load balancing

[nsoft]

Opening this ticket as a place to stash thoughts related to balancing load across nodes (once we have the ability to start multiple nodes that talk to each other!). Of course others should feel free to comment!

Initial thought: Replicate or even directly use parts of the Solr autoscaling framework. It looks reasonably easy to understand, and if our scaling has a familiar feel to Solr's that's a plus for search engineers who are expected to be our primary users.