feat(cache): add iterate method to list files from cache

[Altahrim]

Summary

Add an iterator to list files from a directory

Reduce memory usage on large directory listing (≈ 800 files here):

Also helps to remove large number of files with Primary object store

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[artonge]

Given that getFolderContentsById() does not support those parameters, are $includeMetadata and $sortByName really useful?

Also, you touched a comment suggesting that having a mimetype parameter would be nice.

[Altahrim]

These parameters are optimisations for the underlying query. If we don't use metadata, better avoid the join. If we don't need sorted elements, better leave default order.

From what I've found, the mimetype filter is not really used. That's why I didn't implemented it, but I can :)

[Altahrim]

Maybe we should think about another kind of API… Something like:

// Similar to getFolderContentsById()
$cache->iterateById($folderId)->withMetadata()->sortBy(ICache::SORT_NAME)->exec();
// Unsorted list of ID
$cache->iterateById($folderId)->asType(ICache::RESULT_ID)->exec();

[artonge]

Another though, adding those kinds of arguments/filters sounds more like a search than a listing. So if those are actually useful, maybe we could name the new method something like searchFolderContentsById.