[3183] Record timestamp of the last Renew and one other trivial fix.

tests/tools/perfdhcp/stats_mgr.h

@@ -1169,6 +1169,8 @@ public:
             return("SOLICIT-ADVERTISE");
         case XCHG_RR:
             return("REQUEST-REPLY");
+        case XCHG_RN:
+            return("RENEW-REPLY");
         default:
             return("Unknown exchange type");
         }

tests/tools/perfdhcp/test_control.cc

@@ -1491,6 +1491,7 @@ TestControl::sendDiscover4(const TestControlSocket& socket,
 
 bool
 TestControl::sendRenew(const TestControlSocket& socket) {
+    last_renew_ = microsec_clock::universal_time();
     Pkt6Ptr reply = reply_storage_.getRandom();
     if (!reply) {
         return (false);