cubicweb-fastimport #4821873 If there is no worker, run immediately the deferred hooks [open]

Running an import script on a database with no instance running obliges to run the tasks manually. It should be automatic so that we do not have bother with that.

I suggest, in this case, not to rely at all on a CWWorkerTask and the workercube, which would lower the adoption cost of fastimport.

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