Adding some debug output to NetworkConnection for the next time we run into the endless

Adding some debug output to NetworkConnection for the next time we run into the endless
message loop... :(
Starting core with --sputdev will print a warning whenever a message is received twice from the socket..
That will make crystal-clear if the bug is in our handler or in QTcpSocket.
NOTE: Starting core with --sputdev currently has no other side-effects, don't try this with the client though :)