preparation for performance improvement by asking server for existence of objects rather than trying to fetch them

[davidfirst]

This will be actually used once remotes are updated and support the new GraphQL query.
An example of this query:

query HasObjects {
  scope {
    hasObjects(hashes: ["075b5e1d5ab0c81e6a17648cbe966091c8d9bfb1", "075b5e1d5ab0c81e6a17648cbe966091c8d9bfb2"])
  }
}

and a result could be:

{
  "data": {
    "scope": {
      "hasObjects": [
        "075b5e1d5ab0c81e6a17648cbe966091c8d9bfb1"
      ]
    }
  }
}