Refactor SessionMetadata and TransportCredentials toString() methods

This cleanups up the toString() methods of all implementations of
these interfaces, as pre-work for adding tests against the legacy
logging statement in FlowRunner used for ICANN reporting, so that we
can validate against any changes to that log statement in the future.

It removes system hash codes since those aren't really safe to rely on
in test code and they really don't help with debugging anyway.  It
also standardizes SessionMetadata.toString() a bit and regroups
methods on that interface so all the getters are together.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125686039

java/google/registry/flows/GaeUserCredentials.java

@@ -15,8 +15,8 @@
 package google.registry.flows;
 
 import static com.google.appengine.api.users.UserServiceFactory.getUserService;
+import static com.google.common.base.MoreObjects.toStringHelper;
 import static com.google.common.base.Strings.nullToEmpty;
-import static java.lang.System.identityHashCode;
 
 import com.google.appengine.api.users.User;
 import com.google.common.annotations.VisibleForTesting;
@@ -59,7 +59,9 @@ public class GaeUserCredentials implements TransportCredentials {
 
   @Override
   public String toString() {
-    return String.format("GaeUserCredentials@%s{gaeUser: %s}", identityHashCode(this), gaeUser);
+    return toStringHelper(getClass())
+        .add("gaeUser", gaeUser)
+        .toString();
   }
 
   /** User is not logged in as a GAE user. */