Allow queue size and concurrency limits to be overridden

Fixed sizes faired well to start, but it's now proven useful to be able to increase these limits in a controlled fashion.

Fixes #122

includes/constants.php

@@ -3,13 +3,22 @@
 namespace Automattic\WP\Cron_Control;
 
 /**
- * Job queues
+ * Adjustable queue size and concurrency limits, to facilitate scaling
+ */
+$job_queue_size = defined( 'CRON_CONTROL_JOB_QUEUE_SIZE' ) && is_numeric( \CRON_CONTROL_JOB_QUEUE_SIZE ) ? absint( \CRON_CONTROL_JOB_QUEUE_SIZE ) : 10;
+define( __NAMESPACE__ . '\JOB_QUEUE_SIZE', $job_queue_size );
+unset( $job_queue_size );
+
+$job_concurrency_limit = defined( 'CRON_CONTROL_JOB_CONCURRENCY_LIMIT' ) && is_numeric( \CRON_CONTROL_JOB_CONCURRENCY_LIMIT ) ? absint( \CRON_CONTROL_JOB_CONCURRENCY_LIMIT ) : 10;
+define( __NAMESPACE__ . '\JOB_CONCURRENCY_LIMIT', $job_concurrency_limit );
+unset( $job_concurrency_limit );
+
+/**
+ * Job runtime constraints
  */
-const JOB_QUEUE_SIZE              = 10;
 const JOB_QUEUE_WINDOW_IN_SECONDS = 60;
 const JOB_TIMEOUT_IN_MINUTES      = 10;
 const JOB_LOCK_EXPIRY_IN_MINUTES  = 30;
-const JOB_CONCURRENCY_LIMIT       = 10;
 
 /**
  * Locks