Fetching photo albums is slow because of many albums without covers

[ColonelPhantom]

What?

Pages that fetch photo albums (/members/photos and /api/v2/photos/albums) are often slow, because they need to fetch cover images. When no cover image is set, two(!) queries are performed, one to check if there are photos (???) and one to select a random image.

Why?

Performance intensive; /api/v2/photos/albums often takes over 1 second.

How?

At the very least, do not use the exists check since it should always pass. Preferably, do something to cover images, e.g. make we could maybe make it non-optional?

[DeD1rk]

#3889 makes the situation much better. For a real solution, I suggest we just save the cover that would be selected randomly to the _cover field in Album.save(). (no need to change the schema as it probably should have SET_NULL).