reconstruct deferred queries with knowledge about fragments

[Geal]

Fix #2105
Fix #2099

When we are using @defer, response formatting must apply on a subset of the query (primary or deferred), that is reconstructed from information provided by the query planner: a path into the response and a subselection. Previously, the query planner did not add fragments to that path, which resulted in reconstruction issues for queries like:

query {
  me {
    ... on User {
      id
      fullName
      memberships {
        permission
        account {
          ... on Account @defer {
            name
          }
        }
      }
    }
  }
}

we would get a deferred part with the path /me/memberships/account and the subselection { name }. The path does not give enough information to reconstruct the query when me returns an interface: we cannot know which type is used.

The query planner 2.2.2 introduces a fragment in the path to solve that, like /me/... on User/memberships/account. It also removes the 'flatten' element from deferred nodes paths, so the code has to be adapted a bit.

[Geal]

asking for a review on this before it is done, because the code is finished but it's waiting for a query planner and router-bridge release