chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/classes/abstracts/ActionScheduler_Abstract_Schedule.php

@@ -0,0 +1,89 @@
+<?php
+
+/**
+ * Class ActionScheduler_Abstract_Schedule
+ */
+abstract class ActionScheduler_Abstract_Schedule extends ActionScheduler_Schedule_Deprecated {
+
+	/**
+	 * The date & time the schedule is set to run.
+	 *
+	 * @var DateTime
+	 */
+	private $scheduled_date = null;
+
+	/**
+	 * Timestamp equivalent of @see $this->scheduled_date
+	 *
+	 * @var int
+	 */
+	protected $scheduled_timestamp = null;
+
+	/**
+	 * Construct.
+	 *
+	 * @param DateTime $date The date & time to run the action.
+	 */
+	public function __construct( DateTime $date ) {
+		$this->scheduled_date = $date;
+	}
+
+	/**
+	 * Check if a schedule should recur.
+	 *
+	 * @return bool
+	 */
+	abstract public function is_recurring();
+
+	/**
+	 * Calculate when the next instance of this schedule would run based on a given date & time.
+	 *
+	 * @param DateTime $after Start timestamp.
+	 * @return DateTime
+	 */
+	abstract protected function calculate_next( DateTime $after );
+
+	/**
+	 * Get the next date & time when this schedule should run after a given date & time.
+	 *
+	 * @param DateTime $after Start timestamp.
+	 * @return DateTime|null
+	 */
+	public function get_next( DateTime $after ) {
+		$after = clone $after;
+		if ( $after > $this->scheduled_date ) {
+			$after = $this->calculate_next( $after );
+			return $after;
+		}
+		return clone $this->scheduled_date;
+	}
+
+	/**
+	 * Get the date & time the schedule is set to run.
+	 *
+	 * @return DateTime|null
+	 */
+	public function get_date() {
+		return $this->scheduled_date;
+	}
+
+	/**
+	 * For PHP 5.2 compat, because DateTime objects can't be serialized
+	 *
+	 * @return array
+	 */
+	public function __sleep() {
+		$this->scheduled_timestamp = $this->scheduled_date->getTimestamp();
+		return array(
+			'scheduled_timestamp',
+		);
+	}
+
+	/**
+	 * Wakeup.
+	 */
+	public function __wakeup() {
+		$this->scheduled_date = as_get_datetime_object( $this->scheduled_timestamp );
+		unset( $this->scheduled_timestamp );
+	}
+}