Core/Auth: Mitigate DoS attacks to authserver

Mitigate DoS attacks to authserver like "Wow Auth Flooder.exe" by allowing a finite number of AUTH_LOGON_CHALLEGE packets in a row from same socket, 3 sounds like a reasonable limit.

src/server/authserver/Server/AuthSocket.cpp

@@ -222,12 +222,25 @@ void AuthSocket::OnClose(void)
 // Read the packet from the client
 void AuthSocket::OnRead()
 {
+    #define MAX_AUTH_LOGON_CHALLENGES_IN_A_ROW 3
+    uint32 challengesInARow = 0;
     uint8 _cmd;
     while (1)
     {
         if (!socket().recv_soft((char *)&_cmd, 1))
             return;
 
+        if (_cmd == AUTH_LOGON_CHALLENGE)
+        {
+            ++challengesInARow;
+            if (challengesInARow == MAX_AUTH_LOGON_CHALLENGES_IN_A_ROW)
+            {
+                TC_LOG_WARN(LOG_FILTER_AUTHSERVER, "Got %u AUTH_LOGON_CHALLENGE in a row from '%s', possible ongoing DoS", challengesInARow, socket().getRemoteAddress().c_str());
+                socket().shutdown();
+                return;
+            }
+        }
+
         size_t i;
 
         // Circle through known commands and call the correct command handler