Refactor blobstor configuration

[fyrchik]

Make blobstor configuration contain multiple same-level "blobstors", each with it's own policy.

blobstor:
  -  type: blobovnicza.tree
     path: /srv/neofs/small0
     size: 4g
     depth: 1
     width: 4
     opened_cache_capacity: 50
     policy:
       max_object_size: 1024k
  -  type: fstree
     path: /srv/neofs/big0
     compress: true
     perm: 0644
     depth: 5

• Put fstree and blobovnicza tree configuration on a single level #1523
• Reuse bolt DB configuration between metabase, writecache, blobovnicza #1524
• Add a policy for filtering objects by type #1525
• Add single bolt DB storage type (used in writecache, could be used for auxilliary objects) #1526
• Write a sanity check (every object should be places in at least one storage type) or disallow specifying a policy for the last type of storage or mark some as default. #1527
• Make writecache configuration similar to the blobstor one (however, last storage can have some policy, some objects are not stored in writecache). #1528

[roman-khimov]

In general, superseded by #2497. Some inner ones can still be useful though.