Support replicas in a slice

Currently, only RAID 0+1 like fragile cluster is supported.
For safety, each slice in slices should be replicable, like RAID 1+0.
(However, top-level replicas must be there even if RAID 1+0 like structure is supported, because it is useful to migrate an existing slice to new one.)

ex:

  replicas: [
    { // replica
       slices: [
        { // slice
          replicas: [
            { volume: ... }, // replica of a slice
            { volume: ... }
          ]
        }
      ]
    }
  ]

See also: http://itpro.nikkeibp.co.jp/article/Watcher/20060518/238330/