Race condition when processing new RFD

[augustuswm]

If two jobs for a never before seen RFD number get processed in the same batch there is nothing to prevent them from both creating a new RFD for the new number. This should be enforced at the db level by a unique index.

How this should be handled at the processor level is undecided. It should likely be an error and cause the job to fail. To prevent jobs from getting to this place there likely needs to be some kind of filtering for jobs that are going to create new rfd entries such that two new rfd jobs with the same rfd number never run in the same batch.