Make cron schedule configurable

[wpscholar]

Having a 20-second cron schedule could overwhelm the site’s resources with overlapping cron tasks on a high-traffic site.

While I realize this 20-second cron is important when setting up a site to ensure plugins are properly installed during onboarding, I don't think we should retain this cron schedule past onboarding.

wp-module-tasks/includes/Scheduler.php

Lines 36 to 59 in a7d309c

	/**
	* Add a 20 seconds interval
	*
	* @param array $schedules The existing interval schedules
	*/
	public function add_interval_schedule( $schedules ) {
	// Adds the schedule for the given intervals in seconds
	if ( ! array_key_exists( 'twenty_seconds', $schedules ) || 20 !== $schedules[ 'twenty_seconds' ]['interval'] ) {
	$schedules[ 'twenty_seconds' ] = array(
	'interval' => 20,
	'display' => __( 'Cron to run once every twenty seconds' ),
	);
	}
	// Adds the schedule for the given intervals in seconds
	if ( ! array_key_exists( 'ten_minutes', $schedules ) || 600 !== $schedules[ 'ten_minutes' ]['interval'] ) {
	$schedules[ 'ten_minutes' ] = array(
	'interval' => 600,
	'display' => __( 'Cron to run once every ten minutes' ),
	);
	}
	return $schedules;
	}

The existing cleanup task should happen on the pre-existing 15-minute cron instead of adding a new 10-minute cron.

We should remove the 20-second cron and make it possible to configure custom cron schedules to handle different groups/types of tasks at different intervals.

[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.