Decouple SessionMetadata and TransportCredentials

TransportCredentials are per-request, not per-session, and
there's no reason to carry them within SessionMetadata.

While I'm in here, get rid of "null" credentials.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125202213

java/google/registry/flows/PasswordOnlyTransportCredentials.java

@@ -0,0 +1,28 @@
+// Copyright 2016 The Domain Registry Authors. All Rights Reserved.
+//
+// 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 google.registry.flows;
+
+import google.registry.flows.EppException.AuthenticationErrorException;
+import google.registry.model.registrar.Registrar;
+
+/** A transport credentials that validates the registrar's EPP password and nothing else. */
+public class PasswordOnlyTransportCredentials implements TransportCredentials {
+  @Override
+  public void validate(Registrar r, String password) throws AuthenticationErrorException {
+    if (!r.testPassword(password)) {
+      throw new BadRegistrarPasswordException();
+    }
+  }
+}