Adds background task which automatically removes zone bundles

- Adds a background task to the sled agent, running
  `zone_bundle::cleanup_task`. That periodically combs through all the
  zone bundles, and removes "low priority" ones to remain within a
  percentage of the storage dataset quota.
- Adds tools for controlling the priority of a bundle. This currently
  sorts bundles by their timestamp and cause, where for example
  explicitly requested bundles have the highest priority.
- Adds RPC-like mechanism for asking the cleanup task to report usage
  information, the number of bytes consumed in the storage directories,
  for zone bundles.
- Adds RPC and sled-agent API for updating the cleanup context,
  including period of auto cleanups, priority order, and the percentage
  of the dataset quota allowed.
- Adds RPC and sled agent API for making explicit request to trigger a
  cleanup.

Author: bnaecker