Consider making priority optional

[wpscholar]

It seems like there would be times when just ordering tasks by priority alone could be limiting. It seems like we might want to have some tasks processed based on date/time, with priority being optional or being an additional sort if priority is the same.

I can see not using priority at all for handling things like processing admin page view events and instead handling those as either a stack (LIFO) or a queue (FIFO).

There could be other times when we would want a true queue or stack instead of a solely priority-based system.

I'd love to see a system where you can instantiate a queue, and configure it to run one or more jobs on a subset of tasks (e.g. by task name) at a specific cron interval. Likewise, you could instantiate a stack or priority queue at different intervals for different subsets of tasks.

I think this package would provide some inspiration in this direction: https://github.com/deliciousbrains/wp-queue