net: Add and document network messages in protocol.h

- Avoids string typos (by making the compiler check)
- Makes it easier to grep for handling/generation of a certain message type
- Refer directly to documentation by following the symbol in IDE
- Move list of valid message types to protocol.cpp:
    protocol.cpp is a more appropriate place for this, and having
    the array there makes it easier to keep things consistent.

Github-Pull: #7181
Rebased-From: 9bbe71b641

src/net.cpp

@@ -459,7 +459,7 @@ void CNode::PushVersion()
         LogPrint("net", "send version message: version %d, blocks=%d, us=%s, them=%s, peer=%d\n", PROTOCOL_VERSION, nBestHeight, addrMe.ToString(), addrYou.ToString(), id);
     else
         LogPrint("net", "send version message: version %d, blocks=%d, us=%s, peer=%d\n", PROTOCOL_VERSION, nBestHeight, addrMe.ToString(), id);
-    PushMessage("version", PROTOCOL_VERSION, nLocalServices, nTime, addrYou, addrMe,
+    PushMessage(NetMsgType::VERSION, PROTOCOL_VERSION, nLocalServices, nTime, addrYou, addrMe,
                 nLocalHostNonce, strSubVersion, nBestHeight, !GetBoolArg("-blocksonly", DEFAULT_BLOCKSONLY));
 }