indexer: serialization issue on latest deployed indexer

There is still a serialization issue.

[2018-10-25 19:23:28,670: ERROR/MainProcess] Task swh.indexer.tasks.OrchestratorAllContents[410b4fe2-f036-421c-a4fa-7d2bdee94feb] raised unexpected: EncodeError(TypeError(TypeError('<GroupResult: c2d43b89-eda2-47f2-9af4-9e81cb8d8c17 [2c339c3d-2d47-4ab6-92c6-67ecefe9c171]> is not JSON serializable',),),)
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 438, in __protected_call__
    return self.run(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/swh/scheduler/task.py", line 163, in run
    self.send_event('task-result', result=result)
  File "/usr/lib/python3/dist-packages/celery/app/task.py", line 803, in send_event
    return d.send(type_, uuid=req.id, **fields)
  File "/usr/lib/python3/dist-packages/celery/events/__init__.py", line 238, in send
    self.publish(type, fields, self.producer, blind)
  File "/usr/lib/python3/dist-packages/celery/events/__init__.py", line 215, in publish
    headers=self.headers,
  File "/usr/lib/python3/dist-packages/kombu/messaging.py", line 165, in publish
    compression, headers)
  File "/usr/lib/python3/dist-packages/kombu/messaging.py", line 241, in _prepare
    body) = dumps(body, serializer=serializer)
  File "/usr/lib/python3/dist-packages/kombu/serialization.py", line 164, in dumps
    payload = encoder(data)
  File "/usr/lib/python3.5/contextlib.py", line 77, in __exit__
    self.gen.throw(type, value, traceback)
  File "/usr/lib/python3/dist-packages/kombu/serialization.py", line 59, in _reraise_errors
    reraise(wrapper, wrapper(exc), sys.exc_info()[2])
  File "/usr/lib/python3/dist-packages/kombu/five.py", line 131, in reraise
    raise value.with_traceback(tb)
  File "/usr/lib/python3/dist-packages/kombu/serialization.py", line 55, in _reraise_errors
    yield
  File "/usr/lib/python3/dist-packages/kombu/serialization.py", line 164, in dumps
    payload = encoder(data)
  File "/usr/lib/python3/dist-packages/anyjson/__init__.py", line 141, in dumps
    return implementation.dumps(value)
  File "/usr/lib/python3/dist-packages/anyjson/__init__.py", line 89, in dumps
    raise TypeError(TypeError(*exc.args)).with_traceback(sys.exc_info()[2])
  File "/usr/lib/python3/dist-packages/anyjson/__init__.py", line 87, in dumps
    return self._encode(data)
  File "/usr/lib/python3/dist-packages/simplejson/__init__.py", line 380, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python3/dist-packages/simplejson/encoder.py", line 291, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3/dist-packages/simplejson/encoder.py", line 373, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python3/dist-packages/simplejson/encoder.py", line 268, in default
    raise TypeError(repr(o) + " is not JSON serializable")
kombu.exceptions.EncodeError: <GroupResult: c2d43b89-eda2-47f2-9af4-9e81cb8d8c17 [2c339c3d-2d47-4ab6-92c6-67ecefe9c171]> is not JSON serializable

dashboard: http://kibana0.internal.softwareheritage.org:5601/app/kibana#/dashboard/289ce780-d88d-11e8-b8ce-cf95f437ce37

Related: swh/meta$323

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Migrated from T1291 (view on Phabricator)