Only `net.TCPConn` supports `SetKeepAlive` and `SetKeepAlivePeriod`
by default, so if you want to warp multiple layers of net.Listener,
the `keepaliveListener` should be the one which is closest to the
original `net.Listener` implementation, namely `TCPListener`.
Signed-off-by: Benjamin Wang <wachao@vmware.com>