Add scheduling strategy to execute latest slot only

[ashf72]

@manuel is it possible to add scheduling strategy that will run only the latest of ready slots and will not run the rest? This would be really useful for jobs that do not use slot datetime for data selection, like Conflux. Currently, after the failure, we have a lot of ready slots, while, in fact, only the latest is needed to resume normal operation.
Thank you.

[manuel]

@ashf72 Yes, this is quite easy to do. I'll keep you posted.