ext/session: add recursive session cleanup for dirname > 0

[jorgsowa]

When dirname is bigger than 0, there is no automatic cleanup of old sessions. I think it's oversight and shouldn't work like this. If the performance is the issue then lower GC probability should be set instead ignoring cleanup.

This is not security issue, because documentation clearly state the current behavior, but still should work differently from the first place.

[Girgias]

I don't really understand why we need to loops that are basically identical? Also I'd prefer if those new GC tests are in a subfolder (maybe even a mod_files subfolder)

[jorgsowa]

I don't really understand why we need to loops that are basically identical?

1. First loop is for checking and removing the session when remaining_depth == 0, because this is the only level where session files are stored.

   Second loop is for getting directories based on the value of dirname, becaue this is the level of nesting for the directories where session files are stored.

   But of course, we can merge both of them and check inside what the value of remaining_depth is.

2. New tests have been moved to a new directory

[Girgias]

This looks sensible, not really familiar with C IO operations. @devnexen could you have a quick look? :)

[devnexen]

I had a good look, it looks correct ; just need to address gina's comment.