[Sheepdog] [PATCH v4 12/12] collie: enable cluster-wide snapshot command

[Christoph Hellwig]

> $collie cluster snapshot # snapshot the whole cluster
> $collie cluster snapshot -l # list user snapshot info
> $collie cluster snapshot -R epoch # restore to state of targeted snapshot
> 
> For e.g. below script
> #!/bin/bash

This whole stuff should go into doc/, including a bit more text on
what this whole operation is useful for, and in what case you would run
it.

> +++ b/collie/cluster.c
> @@ -15,11 +15,14 @@
>  #include <sys/time.h>
>  
>  #include "collie.h"
> +#include "../sheep/farm.h"

Please make sure all constants that collie would need go into the
protocol header, including a sheep/farm specific header here defeats
the abstraction barrier.