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/net.h

@@ -101,6 +101,20 @@ class CTransaction;
 class CNodeStats;
 class CClientUIInterface;
 
+struct CSerializedNetMsg
+{
+    CSerializedNetMsg() = default;
+    CSerializedNetMsg(CSerializedNetMsg&&) = default;
+    CSerializedNetMsg& operator=(CSerializedNetMsg&&) = default;
+    // No copying, only moves.
+    CSerializedNetMsg(const CSerializedNetMsg& msg) = delete;
+    CSerializedNetMsg& operator=(const CSerializedNetMsg&) = delete;
+
+    std::vector<unsigned char> data;
+    std::string command;
+};
+
+
 class CConnman
 {
 public: