[Sheepdog] [PATCH 1/2] sheep: sheep: handle node change event first

[Liu Yuan]

Hi Kazum,

	We test this two changes heavily without unexpected behaviour (such as
data loss, inconsistent epoch view, blocked collie operations).
	But I am still uncertain if there are any other bad side effect we
don't consider. I carefully examined the code and didn't find logic flaw
with these two changes.

	How do you think of it?

Thanks,
Yuan