discovery: don't use struct embedding in SharedUDPConn

[nkryuchkov]

If the implementation of go-ethereum changes and uses net.UDPConn's methods other than ReadFromUDPAddrPort, struct embedding will call net.UDPConn's method directly and the Unhandled channel won't get drained. This PR would break the compilation in such a case because SharedUDPConn won't satisfy the internal go-ethereum's interface