Add extended indexes for hook_status_scheduled_date_gmt and status_sheduled_date_gmt #992

…heduled_date_gmt Fixes #978 The current `hook` and `status` indexes can be pretty useless as there isn't enough uniqueness in the generated index to add much value to query performance. Extending these keys to include the `status` and `scheduled_date_gmt` make them much more useful for functions like `as_next_scheduled_action()` and `ActionScheduler_Store::has_pending_actions_due()`.