Refactor the keepAliveListener and keepAliveConn

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>

server/embed/etcd.go

@@ -666,12 +666,6 @@ func configureClientListeners(cfg *Config) (sctxs map[string]*serveCtx, err erro
 			sctx.l = transport.LimitListener(sctx.l, int(fdLimit-reservedInternalFDNum))
 		}
 
-		if network == "tcp" {
-			if sctx.l, err = transport.NewKeepAliveListener(sctx.l, network, nil); err != nil {
-				return nil, err
-			}
-		}
-
 		defer func(u url.URL) {
 			if err == nil {
 				return