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.
- Adds a bunch of tests around the priority ordering and cleanup RPC
  mechanics

Wrap zone bundling into a single type

- Adds the `ZoneBundler` for managing the creation and listing of
  zones, and running background cleanup task. This is created pretty
  early, by the `StorageManager`, and cloned to the other objects that
  need access to bundles.
- Move public free functions from the `zone_bundle` crate into methods
  on the `ZoneBundler`, and call them from the instance / service
  managers and sled agent.
- Make the "list-all-zones" endpoint in the sled agent accept a filter
  directly and propagate to the listing routines inside the bundler.
  This removes filtering on the client.
- Additional tests around listing / filtering.

Review feedback

- Add more error variants, to better maintain context and the actual
  error chains.
- Remove zone-specific commmands
- Remove hand-rolled `PartialOrd` implementation for simplicity, but add
  comment about the variant order being load-bearing.
- Add the handle for the cleanup task to the `ZoneBundler`, and abort it
  on drop.

Author: bnaecker