Add support for deletion tasks

For data scheduling to be effective, it becomes imperative for administrative clients such as SLURM to be able to delete resources after they have been successfully copied elsewhere.

Currently, transferors only support the IOTASK_COPY and IOTASK_MOVE options (and the latter is yet unimplemented). It would be very useful to include and IOTASK_DELETE that effectively deletes a resource from a namespace.