AuditFilesNotInDatabase uses maximum records before actually checking if a file exists

Problem/Motivation

When checking for files that are on disk, but not in the database, the code iterates through the subdirectories to get a list of files. When the maximum number of files has been found, the code stops looking. This seems useful to prevent issues with large file directories, however, existing files are also counted in the maximum. If you have more then the number of files specified in the maximum, a lot of files are never checked.

Steps to reproduce

Add 20 files in the checked directory.
Set the maximum number of files to 15.
Observe 5 files not being checked.

Proposed resolution

Not sure what would be the best way to approach this. The maximum records should at least check if a file exists first and only count the ones that do not exist. To prevent performance issues we might want to check per subfolder?

Remaining tasks

Discuss the best solution for the issue.
Write a patch.
Add tests.
Commit.

User interface changes

Maybe allow checking files per subfolder?

API changes

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Data model changes

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