ALTO/METS: Store results in an SQLite database instead of API

This would affect both the ALTO and the METS upload.

Accept a new optional argument --db, a path to the exported SQLite database. Changes:

• Do not load an API client, instead initialize the DB with all tables
• Do not check for presence of parent element, create it in the DB (type="folder")
• Call open_database
• Make the client argument optional everywhere
• If client is None, do SQLite queries instead of API calls