Qless should not allow both a recurring and non-recurring job with the same jid

[myronmarston]

We had a case where we used default_job_options to assign a generated, idempotent jid to a job based on the args. Usually we enqueue it as a recurring job but sometimes we've had to enqueue one-off individual jobs. I realized recently that this was putting qless into a slightly inconsistent state; qless.jobs[the_jid] now returned the one-off job rather than the recurring job even though the recurring job was still there (with that same jid) in Qless.

It would be nice if Qless would fail loudly if you try Queue#put with a jid that currently belongs to a recurring job, or Queue#recur with a jid that currently belongs to a non-recurring job.