Add a cli command to get statistics for a given config entry

As well as a command to list partitions being checked.

For example:

$ swh scrubber check stats snapshot_16 -j
{
  "config": {
    "name": "snapshot_16",
    "datastore": {
      "package": "storage",
      "cls": "postgresql",
      "instance": "postgresql:///?service=swh-storage"
    },
    "object_type": "snapshot",
    "nb_partitions": 65536,
    "check_hashes": true,
    "check_references": true
  },
  "min_duration": 0.002196,
  "max_duration": 0.107398,
  "avg_duration": 0.005969,
  "checked_partition": 65536,
  "running_partition": 0,
  "missing_object": 0,
  "missing_object_reference": 0,
  "corrupt_object": 0
}

$ swh scrubber check running cfg1

Running partitions for cfg1 [id=1, type=snapshot]:
0:	running since today (20 minutes)

This is in particular to help writing integration tests.