Adds background task which automatically removes zone bundles (#3868)

- 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