[FAB-4557] gossip: Do not sign block messages

In gossip we currently sign every message in the following way:
- Message sent by the Gossip() API call - uses the peer's signing identity
  to sign the message.
- Message sent via Send() is signed in a NOOP manner (the signature is empty)

Since blocks are already signed by the ordering service
there is no point in signing them with the peer's signing identity.
This commit makes such message be NOOP-signed.

Change-Id: I0e5dca73e2ba3e711955829435172ed7f0365b61
Signed-off-by: Yacov Manevich <yacovm@il.ibm.com>

gossip/gossip/gossip_impl.go

@@ -629,9 +629,15 @@ func (g *gossipServiceImpl) Gossip(msg *proto.GossipMessage) {
 		GossipMessage: msg,
 	}
 
-	_, err := sMsg.Sign(func(msg []byte) ([]byte, error) {
-		return g.mcs.Sign(msg)
-	})
+	var err error
+	if sMsg.IsDataMsg() {
+		sMsg, err = sMsg.NoopSign()
+	} else {
+		_, err = sMsg.Sign(func(msg []byte) ([]byte, error) {
+			return g.mcs.Sign(msg)
+		})
+	}
+
 	if err != nil {
 		g.logger.Warning("Failed signing message:", err)
 		return