Update expected catalog fragments for future V3 catalog
Restructure classes around Volume.

Now, there are 3 type volumes:

 1. SingleVolume, is a physical computer.
 2. SlicesVolume, is a collection of <Slice>s.
 2. ReplicasVolume, is a collection of volumes.

A Slice must have one of volumes.
For example:

  datasets:{
    Default:{
     replicas:[
       {
         slices:[
           { volume: { address:... } },
           {
             volume: {
               replicas: [
                 { address: ... },
                 { address: ... }
               ]
             }
           },
           {
             volume: {
               slices: [
                 { volume: { address:... } },
                 { volume: { address:... } },
                 {
                   volume: {
                     replicas: [
                       { address:... },
                       { address:... }
                     ]
                   }
                }
              ]
            }
          }
        ]
      }
    ]
  }}

It will be parsed as:

  ReplicasVolume:[
    Slices:[
      Slice:SingleVolume,
      Slice:ReplicasVolume:[
        SingleVolume,
        SingleVolume
      ],
      Slice:SlicesVolume:[
        Slice:SingleVolume,
        Slice:SingleVolume,
        Slice:ReplicasVolume:[
          SingleVolume,
          SingleVolume
        ]
      ]
    ]
  ]

Note, currently the top level ReplicasVolume is the Dataset itself.
It is dangerous, so we should change the structure like:

  datasets:{
    Default:{
      volume:{
        replicas:[...]
      }
    }
  }

This breaks backward compatibility - v2 catalogs have to be migrated when they are parsed.
So, after the change, catalog's spec version must be incremented to 3.