Add information about locks to the API

[msm-code]

Probably the easiest way is to add this to the topology endpoint. Instead of returning:

{
    "result": {
        "datasets": {
            "507718ac": {
                "file_count": 50401,
                "indexes": [
                    {
                        "size": 3117031264,
                        "type": "gram3"
                    }
                ],
                "size": 3117031264,
                "taints": []
            }
        }
    },
    "type": "topology"
}

We could add something like "locked_by": [task_id].

In fact, maybe we should also add iterators and their locks to this endpoint (but that's another issue).