Build WorkerActivity instances in an async task

Depends on #697 (closed)

1. add an initialisation state to DataImport:
   • using an Enum: created|available|error
   • default to created
2. create a new task that build up all the initial WorkerActivity for a given process
3. at the end of the task set the state to available or error
4. when worker activity is not needed, set the init state to available