To more closely align return value with other date APIs in WC, namely
WC_Data. Includes changing return value to:
1. a WC_DateTime instead of a timestamp when a scheduled occurrence is found
2. null instead of false, similar to the default of WC_Data::set_date_prop()
To provide an agreed set of public methods that need to be defined
for any class wanting to act as WooCommerce's queue handler.
The require APIs support both delay jobs, by providing APIs to
schedule an action/job/event for some time in the future, and a
non-delayed job API, to add an event to the queue to run as soon
as possible.
Brent Shepherd