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.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125202213

java/google/registry/flows/EppConsoleAction.java

@@ -41,7 +41,8 @@ public class EppConsoleAction implements Runnable {
   @Override
   public void run() {
     eppRequestHandler.executeEpp(
-        new HttpSessionMetadata(new GaeUserCredentials(getUserService().getCurrentUser()), session),
+        new HttpSessionMetadata(session),
+        new GaeUserCredentials(getUserService().getCurrentUser()),
         inputXmlBytes);
   }
 }