[Sheepdog] [PATCH 0/2 v2] support sheepdog shutdown and restart

[MORITA Kazutaka]

Hi,

The patchset I sent yesterday was a bit confusing and hard to verify.
So let's start from supporting simple features.

This patchset supports safe cluster restart.
Conditions sheepdog can start automatically and safely are:
 - `shepherd shutdown` was executed when you shut down sheepdog
 - newly added machines have no local epochs and objects

Otherwise collie returns SD_RES_INCONSISTENT_EPOCH error, and you need
to fix manually or run `shepherd mkfs`.

This patchset is against the master branch and included in the next
branch.

Thanks,

Kazutaka Morita