Feature/shortened names check

[infeo]

This PR adds a new check to the health check API: ShortenedNamesCheck

For all directories ending with c9s it checks, if the file name.c9s exists in its hashed & encoded content matches the directory name.

It produces 4 results:

• Valid shortened resource
• Obese result, if the size of name.c9s is bigger than a cryptofs internal limit
• Missing result, if name.c9s is not a regular file or it does not exist
• Mismatch result, if the directory name does not match the hashed & encoded content

For the future this check can be refinded by providing the directory id of the encrypted dir containing the shortened resource, but for this PR it is out of scope and only noted via inline comments.

[infeo]

This feature is feature-complete, so remove those comments

Should I only remove the "TODO" marker or the whole comment?

[overheadhunter]

Should I only remove the "TODO" marker or the whole comment?

The whole comment. Treat the current implementation as final, document future changes in an issue.