cluster/local: don't process leave event of gateway node

We must not call sd_leave_handler() when the gateway node leaves cluster.  It is
because we already processed the event when the node became gateway.

The local driver can pass tests/048 with this patch.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: check whether there is a pending recovery correctly

Comparing current_rinfo->epoch and sys->epoch is not enough because sheep may
start recovery without incrementing sys->epoch (e.g. unplugging a md disk).
Checking next_rinfo is correct and more straightforward.

This fixes an error of tests/056.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>