Move logic to find next event from `wp_next_scheduled()` to `wp_get_scheduled_event()`.

[peterwilsoncc]

The new function for 5.1 wp_get_scheduled_event() accepts the timestamp as an optional argument. If no timestamp is specified the next scheduled event is returned.

If no timestamp is specified, it current makes use of wp_next_scheduled() to determine the timestamp of the next event. This results in running code to get the cron array and determine the key twice.

The logic could be reversed, wp_get_scheduled_event() could determine the next event and wp_next_scheduled() make use of that function if an event is returned.

This will allow the pre_next_scheduled and next_scheduled filters to be deprecated. If the change is made prior to WP 5.1's release, they can be removed.

[peterwilsoncc]

In 45976.2.diff​

• Unit tests for wp_get_scheduled_event()
• Remove tests for removed filter
• bitwise & replaced with && inside condition

[peterwilsoncc]

return false checks added in 45976.3.diff​.

[desrosj]

This makes sense and 45976.3.diff​ looks good. Tested well in my sandbox.

[peterwilsoncc]

In 44693:

Cron: Move logic for finding the next event's timestamp.

Moves the logic for determining when an event will next run from wp_next_scheduled() to wp_get_scheduled_event().

This improves the performance of wp_get_scheduled_event() by avoiding duplicate function calls and object cache hits.

Props peterwilsoncc.
Fixes #45976.