Add code for revision cleanup

[Gared]

This will add two methods for revision cleanup:

• deleteAllRevisions (copies only the latest version and removes the old database entries)
• deleteRevisions (deletes all revisions, but keeps the last x revisions of a pad and rewrites the first revision)

Relates to #6194

There are still some TODOs:

• Remove duplicated code
• Add config to change behaviour of cleanup (and enable/disable)
• Test with a lot of revisions (mainly the key revisions)