Support for limit run option to avoid delay tasks that previous not finished

[softrebel]

Hi,
We are using redbeat in production and manage periodic task dynamically inside tasks. Some of tasks must be send periodically and run one task instance at once. In default, tasks send from redbeat to celery, these are been queued and not applied immediately. This behavior causes some error and manipulated result in our project. We check available solutions to fix our issue and come up with these:

1. add "max running number" in Entry meta keys and check number of running task in reschedule method
2. add expires argument in _maybe_due_kwargs and pass argument in "maybe_due" method.

Are these solutions are ok in redbeat?
If its ok, please let me to develop these features in redbeat.

Thanks.