These are testability/logging improvements spun off from #7475
- Don't log warnings for shutdown errors in compute hook
- Revise logging around heartbeats and reconcile_all so that we aren't
emitting such a large volume of INFO messages under normal quite
conditions.
- Clean up the `last_error` of TenantShard to hold a ReconcileError
instead of a String, and use that properly typed error to suppress
reconciler cancel errors during reconcile_all_now. This is important for
tests that iteratively call that, as otherwise they would get 500 errors
when some reconciler in flight was cancelled (perhaps due to a state
change on the tenant shard starting a new reconciler).
The binary etc were renamed some time ago, but the path in the source
tree remained "attachment_service" to avoid disruption to ongoing PRs.
There aren't any big PRs out right now, so it's a good time to cut over.
- Rename `attachment_service` to `storage_controller`
- Move it to the top level for symmetry with `storage_broker` & to avoid
mixing the non-prod neon_local stuff (`control_plane/`) with the storage
controller which is a production component.
