Make cron schedule configurable

[newfold-labs]

Make cron schedule configurable

[wpscholar]

Ultimately, I think we don't have enough classes to represent all the concepts and dependencies that we really want. I think we should probably have the following:

• Connection - for now, just a DatabaseConnection; handles reading and writing to data source
• QueryFilter - applied to a connection to allow it to pull only a specific subset of all entries (e.g. tasks by name, or if we change how we do things it could be a Job class name)
• QueueType (one of Queue, Stack, PriorityQueue) - Handles prioritization of tasks, is essentially an application of a QueryFilter focused primarily on sorting.
• Schedule - (one of Interval or Event) Handles setting up cron schedules or one-off events. We could have  existing classes that correspond to the pre-existing cron intervals in WordPress (QuarterHourInterval, HourlyInterval, etc)
• Worker - Takes a connection, schedule, queue type, and (optionally) query filter(s). Handles registration of new tasks, processing tasks, checking task status, recording results, decrementing retries, etc. 
• Job/Task - Individual unit of work. We should have a standard way of registering these. I like how the wp-queue package does it. Ideally would be extended for each job type.
• Job/Task Result - Handles storing and fetching of results, mostly standalone, but would rely on a connection type. 

I'm looking at this as inspiration: https://github.com/deliciousbrains/wp-queue

I get that this is a bit of an involved rewrite if we do this, so if we need to circle back to this as a fast follow we can.