Explicitly stating post_status values in get_attendees_from_payment_token()

The previous 'any' setting would miss statuses assigned to post types where exclude_from_search is true.
Renamed get_data_from_payment_token() to get_post_meta_from_payement_token()

The new name is more explicit, so that you know the data is pulled from post meta, without having to read the source.
Adds numberposts param to get_attendees_from_payment_token()

Previous numberposts wasn't set in the get_posts() query, so it would default to the 'posts per page' option in Core, which is unpredictable. The new default is to query for all attendees, but the calling function can specify a different number if needed.
Cached the results of the database query in get_attendees_from_payment_token()