Since it is not possible to dynamically detect if a job is automatically
cancellable or not, a this new attribute is necessary. Moreover, it let
the maintainer of the repo to adjust the behaviour of the auto cancellation
feature to match exactly what he needs.

This is to accomodate prepended modules.

This commits avoiding conflicts between ArchiveTraceWorker
and ArchiveTracesCronWorker by changing the target of the
latter worker.