Improvements to feature listing

[keithmoss]

• [ ] Understand multiprocessing, Pool, and Manager better via this, this, this, and this
• [x] See if there's a better way of passing arguments to threads
• [x] Try without an uplifted account - will impact QPS ramping
• [x] Move QPS and threads to be command-line options
• [x] Handle no access better
• [ ] Add a means of querying multiple tables with the same schema
• [x] Tidy up, move common code (bbox split, find viable bboxes, ...) to a new common code place
• [x] Work out why the AHP test returns 32,123 features for a 32,125 dataset
• [x] Re-add the bbox cache and make the feature lister split and retry on "too expensive" errors.
• [x] Is it worth multithreading the bbox validator too?
• [x] Write to disk ASAP, not memory?

See Issue #22 for test commands.