Fix performance of consistency check and export

Consistency checks take too long when creating a backup on a large db, sometimes longer than the actual export of the data. In particular, scanning the DOM of every single document is a performance killer.

By default, consistency check will now only test if a document's data can be accessed. Extended checks, which will traverse every document to test for DOM errors, can be enabled via a parameter. To reduce IO, documents are now read in the order they are stored in dom.dbx. This results in higher memory consumption, but makes checks 3 times faster.