Initialise earlier on init

When `ALTERNATE_WP_CRON` is defined, WordPress runs `wp_cron()` too early [bug report here](https://core.trac.wordpress.org/ticket/24160).

As a result, the `'action_scheduler_run_queue'` hook is triggered before `ActionScheduler_QueueRunner::init()`
has run, because it is hooked to `'init'` with the default priority in `ActionScheduler::init()`.