[sheepdog] the use case for leave_list/leave_nodes

[Christoph Hellwig]

I'm trying to understand the use case for the leave_list and all code
associated with it.

>From my reading the intention is to allow a cluster to start as long
as all the original nodes tried to join the cluster.  What makes an
original node that tried to join the cluster but failed special over
one that never tried to join?  It's not going to help us with getting
copies from it without a manual recover at least.