[performance] Consistency checks take too long when creating backup on large db: by default, consistency check will now only test if a document's data can be read. Extended checks, which will traverse every document to test for DOM errors, can be enabled via a parameter. To reduce IO, documents are now accessed in the order they are stored in dom.dbx (makes the checks 3 times faster).
Merge pull request #235 from wolfgangmm/develop

Fix performance of consistency check and export
Merge remote-tracking branch 'eXist/develop' into develop