Zoekt indexer: Implement pulling tasks

Proposal

Zoekt indexer is going to receive tasks from the heartbeat API and after completing the task send another request to the callback URL. It should happen async. We should skip tasks for locked repositories without raising an error

Tasks format

TBD

{
  "id": <NODE_ID>,
  "tasks": [
    {
      "name": "index",
      "payload": <PAYLOAD>
    },
    {
      "name": "delete",
      "payload": {
        "RepoId": <REPO_ID>
      }
    }
  ]
}

We'll need to figure something out about delete because currenly the ID is part of the path