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/EppTlsAction.java

@@ -46,7 +46,7 @@ public class EppTlsAction implements Runnable {
     if (!tlsCredentials.hasSni()) {
       logger.warning("Request did not include required SNI header.");
     }
-    eppRequestHandler.executeEpp(new HttpSessionMetadata(tlsCredentials, session), inputXmlBytes);
+    eppRequestHandler.executeEpp(new HttpSessionMetadata(session), tlsCredentials, inputXmlBytes);
   }
 }