macaroons: Let GenerateNewRootKey be applied to all root keys

With this commit, GenerateNewRootKey will regenerate the Default root
key and will then also check if any other root keys exist and regenerate
those as well.

macaroons/store.go

@@ -325,10 +325,34 @@ func (r *RootKeyStorage) GenerateNewRootKey() error {
 		if bucket == nil {
 			return ErrRootKeyBucketNotFound
 		}
+
+		// The default root key should be created even if it does not
+		// yet exist, so we do this separately from the rest of the
+		// root keys.
 		_, err := generateAndStoreNewRootKey(
 			bucket, DefaultRootKeyID, r.encKey,
 		)
-		return err
+		if err != nil {
+			return err
+		}
+
+		// Now iterate over all the other root keys that may exist
+		// and re-generate each of them.
+		return bucket.ForEach(func(k, v []byte) error {
+			if bytes.Equal(k, encryptionKeyID) {
+				return nil
+			}
+
+			if bytes.Equal(k, DefaultRootKeyID) {
+				return nil
+			}
+
+			_, err := generateAndStoreNewRootKey(
+				bucket, k, r.encKey,
+			)
+
+			return err
+		})
 	}, func() {})
 }