Expose "status json" fields individually

Instead of generating a large JSON object every time a client wants to see details on the status of a cluster, clients should be able to request individual fields. This solution:

- Is easier to use, since the client doesn't have to do as much work parsing the response
- Can require the cluster controller to do only the necessary work, instead of generating many fields with each status request that may be ignored by the client
- Allows "status fields" to be provided by a process other than the cluster controller, without the client seeing any difference in behavior. For example the health metrics calculated with #1114 can be retrieved from the proxies instead.

This could be implemented by using a different special key for each status field (e.g. `\xff/\xff/status/cluster/processes/fbb02e841b223fb0f8edf4deb2913d45/uptime_seconds`). The exact format of the special keys would have to be agreed upon beforehand, but the particular format is not important.