Fix workflow freeze when 2 parallel tasks are completed at the same time by 2 different users

Steps to reproduce:

1. deploy the jar provided in the first comment to $NUXEO/nxserver/plugins
2. navigate to the JSF UI
3. navigate to a File document
4. start the workflow SUPTHI-22
5. observe that 2 tasks approve-1 and approve-2 have been created
6. from now, you need 2 different browsers which are open on the Summary view of the File document
7. with browser 1, set a value for "Ma Var" in task "approve-1" and validate
8. with browser 2, set a value for "Ma Deuz Var" in task "approve-2" and validate
9. observe that an error is displayed
10. in the logs, there is this exception

    2021-09-06T18:47:07,976 ERROR [RoutingTaskActionsBean] org.nuxeo.ecm.platform.routing.api.exception.DocumentRouteException: You don't have the permission to set the workflow variable maVar
    org.nuxeo.ecm.platform.routing.api.exception.DocumentRouteException: You don't have the permission to set the workflow variable maVar
    	at org.nuxeo.ecm.platform.routing.core.impl.GraphNodeImpl.setAllVariables(GraphNodeImpl.java:355) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.core.impl.GraphRunner.resume(GraphRunner.java:122) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.core.impl.DocumentRouteElementImpl.resume(DocumentRouteElementImpl.java:84) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.core.impl.DocumentRoutingEngineServiceImpl.resume(DocumentRoutingEngineServiceImpl.java:50) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.core.impl.DocumentRoutingServiceImpl$CompleteTaskRunner.run(DocumentRoutingServiceImpl.java:369) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.core.api.UnrestrictedSessionRunner.runUnrestricted(UnrestrictedSessionRunner.java:119) ~[nuxeo-core-api-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.core.impl.DocumentRoutingServiceImpl.completeTask(DocumentRoutingServiceImpl.java:336) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.core.impl.DocumentRoutingServiceImpl.endTask(DocumentRoutingServiceImpl.java:866) ~[nuxeo-routing-core-10.10-HF49.jar:?]
    	at org.nuxeo.ecm.platform.routing.web.RoutingTaskActionsBean.endTask(RoutingTaskActionsBean.java:235) [nuxeo-routing-web-10.10-HF32.jar:?]
    

From this step, the workflow is stuck and one can only abandon it.

Expected behavior:

• the second task is validated with an exception and the workflow can continue
• or the second task is not validated because of an exception and the user needs to refresh the page to validate its task