Allow storing and retrieving of per-instance data #3
Description
When using etcd, we could store something more complex than just the thread count. The nature of JSON objects is such that this case would be easy to detect for compatibility. With proper phasing, a forward-compatibility release could be introduced which coped with the new format, ahead of actually storing objects in that format.
Rather than storing a numeric value, we would store an object whose workers member was the current number of threads/sub-processes.