p2p: use package rlp to encode messages

Message encoding functions have been renamed to catch any uses.
The switch to the new encoder can cause subtle incompatibilities.
If there are any users outside of our tree, they will at least be
alerted that there was a change.

NewMsg no longer exists. The replacements for EncodeMsg are called
Send and SendItems.

p2p/server_test.go

@@ -149,7 +149,7 @@ func TestServerBroadcast(t *testing.T) {
 	connected.Wait()
 
 	// broadcast one message
-	srv.Broadcast("discard", 0, "foo")
+	srv.Broadcast("discard", 0, []string{"foo"})
 	golden := unhex("66e94d166f0a2c3b884cfa59ca34")
 
 	// check that the message has been written everywhere