initialize messageCounter to 2 instead of verifying later (#1156)

Clean up the messageCounter checks added in #1154. Instead of checking that
messageCounter is still at 2, just initialize it to 2 and only increment for
non-handshake messages. Handshake packets will always be packets 1 and 2.

connection_state.go

@@ -72,6 +72,8 @@ func NewConnectionState(l *logrus.Logger, cipher string, certState *CertState, i
 		window:    b,
 		myCert:    certState.Certificate,
 	}
+	// always start the counter from 2, as packet 1 and packet 2 are handshake packets.
+	ci.messageCounter.Add(2)
 
 	return ci
 }