Cache operational, added as an option which defaults to on. (See description.)

The cache is now operational, controlled by the all_entries option which defaults to false (ie, on). Caching speeds up performance both by reduced demands on CPU and time, and importantly avoiding imposing on sites by flooding them with parse requests.
The next goal is reparsing failed entries and deciding on how to coordinate the updated entries with the backlog plugin: either by removing the entries from backlog's database after creating fresh entries for each failed entry, or updating the data in the database and letting backlog go along its way.