Handle concurrency while initializing activity

Based on backend!1353
Closes #68 (closed)
This would allow to run processes on a similar worker version and element without processing the element twice (that can cause side effects or other errors).
The backend can return a 409_CONFLICT.

• Add tests

Before: After:

tests/test_elements_worker/test_worker.py

@@ -71,7 +71,8 @@ def test_readonly(responses, mock_elements_worker):
 
     out = mock_elements_worker.update_activity("1234-deadbeef", ActivityState.Processed)
 
-    assert out is None
+    # update_activity returns False in very specific cases
+    assert out is True
     assert len(responses.calls) == len(BASE_API_CALLS)
     assert [
         (call.request.method, call.request.url) for call in responses.calls
@@ -265,7 +266,7 @@ def test_start_activity_conflict(
     )
     from arkindex_worker.worker import logger
 
-    logger.warning = mocker.MagicMock()
+    logger.info = mocker.MagicMock()
 
     mock_elements_worker_with_list.run()
 
@@ -279,14 +280,9 @@ def test_start_activity_conflict(
             "http://testserver/api/v1/workers/versions/12341234-1234-1234-1234-123412341234/activity/",
         ),
     ]
-    assert logger.warning.call_args_list == [
-        mocker.call(
-            "Failed to update activity of element 1234-deadbeef to started due to an API error: Either this activity does not exists or this state is not allowed."
-        ),
-        mocker.call(
-            "Cannot start processing element 1234-deadbeef due to a conflict. "
-            "Another process could have processed it with the same version already."
-        ),
+    assert logger.info.call_args_list[:2] == [
+        mocker.call("Processing page Test Page n°1 (1234-deadbeef) (1/1)"),
+        mocker.call("Skipping element 1234-deadbeef as it was already processed"),
     ]