Streaming JSON API: Use same format for archived events as in exercise endpoint

[hurryabit]

Currently, the archived events in the streaming JSON API look like

{ "archived": "#x:y" }

On the /commands/exercise endpoint they look like

{
  "archived": {
    "templateId": "a:B:C",
    "contractId": "#x:y"
  }
}

I would like to get the latter format on the stream API as well. This consistency makes working with the API easier and more predictable.