At present the plan is that to cancel (for example) a job you send a
cancel operation to the job's URL.
Bob Herroitt and I propose that this be changed: you send a cancel
operation to the printer URL giving a JobId as a parameter.
This has the advantage that the client only needs to know one URL for
all operations on a given printer.
This then carries through into GetJobs returning a set of Jobids instead
of a set of Job URLs. A jobid is just an opaque token, the client should
not attempt to use it except as a way of identifying a job back to the
server.