net: add CVectorWriter and CNetMsgMaker

CVectorWriter is useful for overwriting or appending an existing byte vector.

CNetMsgMaker is a shortcut for creating messages on-the-fly which are suitable
for pushing to CConnman.

src/netmessagemaker.h

@@ -0,0 +1,36 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2016 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_NETMESSAGEMAKER_H
+#define BITCOIN_NETMESSAGEMAKER_H
+
+#include "net.h"
+#include "serialize.h"
+
+class CNetMsgMaker
+{
+public:
+    CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn){}
+
+    template <typename... Args>
+    CSerializedNetMsg Make(int nFlags, std::string sCommand, Args&&... args)
+    {
+        CSerializedNetMsg msg;
+        msg.command = std::move(sCommand);
+        CVectorWriter{ SER_NETWORK, nFlags | nVersion, msg.data, 0, std::forward<Args>(args)... };
+        return msg;
+    }
+
+    template <typename... Args>
+    CSerializedNetMsg Make(std::string sCommand, Args&&... args)
+    {
+        return Make(0, std::move(sCommand), std::forward<Args>(args)...);
+    }
+
+private:
+    const int nVersion;
+};
+
+#endif // BITCOIN_NETMESSAGEMAKER_H