Initialise earlier on init to fix issues with ALTERNATE_WP_CRON

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()`.

This commit moves the initialisation to a higher priority to make sure everything it setup and attached to `'action_scheduler_run_queue'` before `wp_cron()` runs.