do not panic on rebalance during server restarts (#19563)

This PR makes a feasible approach to handle all the scenarios
that we must face to avoid returning "panic."

Instead, we must return "errServerNotInitialized" when a
bucketMetadataSys.Get() is called, allowing the caller to
retry their operation and wait.

Bonus fix the way data-usage-cache stores the object.
Instead of storing usage-cache.bin with the bucket as
`.minio.sys/buckets`, the `buckets` must be relative
to the bucket `.minio.sys` as part of the object name.

Otherwise, there is no way to decommission entries at
`.minio.sys/buckets` and their final erasure set positions.

A bucket must never have a `/` in it. Adds code to read()
from existing data-usage.bin upon upgrade.

cmd/bucket-object-lock.go

@@ -44,7 +44,6 @@ func (sys *BucketObjectLockSys) Get(bucketName string) (r objectlock.Retention,
 		if errors.Is(err, errInvalidArgument) {
 			return r, err
 		}
-		logger.CriticalIf(context.Background(), err)
 		return r, err
 	}
 	return config.ToRetention(), nil