[Sheepdog] [PATCH 0/4] fix a race when multiple sheep join a cluster very quickly

[Christoph Hellwig]

This is a pretty big series, with the first three patches beeing preparation
for the last one, which fixes a race that leads to mismatching epochs on
sheep that joined a cluster very quickly next to each other.

It's only been verified on the corosync and local cluster drivers, so
additional testing and review for the others is very welcome.