Move gateway unsupported functions into a common struct. (#5009)

This is done to avoid repeated declaration of not-implemented
functions for each gateway. It also avoids a possible bug in go
https://github.com/golang/go/issues/18468 which is triggered on
our multiple PRs already.

cmd/gateway-unsupported.go

@@ -0,0 +1,59 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import "io"
+
+type gatewayUnsupported struct{}
+
+// CopyObjectPart - Not implemented.
+func (a gatewayUnsupported) CopyObjectPart(srcBucket, srcObject, destBucket, destObject string, uploadID string,
+	partID int, startOffset int64, length int64) (info PartInfo, err error) {
+	return info, traceError(NotImplemented{})
+}
+
+// AnonPutObject creates a new object anonymously with the incoming data,
+func (a gatewayUnsupported) AnonPutObject(bucket, object string, size int64, data io.Reader,
+	metadata map[string]string, sha256sum string) (ObjectInfo, error) {
+	return ObjectInfo{}, traceError(NotImplemented{})
+}
+
+// HealBucket - Not relevant.
+func (a gatewayUnsupported) HealBucket(bucket string) error {
+	return traceError(NotImplemented{})
+}
+
+// ListBucketsHeal - Not relevant.
+func (a gatewayUnsupported) ListBucketsHeal() (buckets []BucketInfo, err error) {
+	return nil, traceError(NotImplemented{})
+}
+
+// HealObject - Not relevant.
+func (a gatewayUnsupported) HealObject(bucket, object string) (int, int, error) {
+	return 0, 0, traceError(NotImplemented{})
+}
+
+// ListObjectsHeal - Not relevant.
+func (a gatewayUnsupported) ListObjectsHeal(bucket, prefix, marker, delimiter string, maxKeys int) (loi ListObjectsInfo, e error) {
+	return loi, traceError(NotImplemented{})
+}
+
+// ListUploadsHeal - Not relevant.
+func (a gatewayUnsupported) ListUploadsHeal(bucket, prefix, marker, uploadIDMarker,
+	delimiter string, maxUploads int) (lmi ListMultipartsInfo, e error) {
+	return lmi, traceError(NotImplemented{})
+}