tonie_json.cache_images downloads pictures while tonies.json loads, although cache_preload is off #570

Describe the bug
With tonie_json.cache_images on and tonie_json.cache_preload off, pictures should only be downloaded when they are
first requested. Since #527 (e57fa45), cache_add() checks for an existing entry with cache_fetch_by_url(), which
also downloads the picture when its file is not in the cache yet. Loading tonies.json therefore downloads pictures one
after the other:

• at start, every picture whose URL tonies.json repeats,
• at every reload (/api/toniesJsonReload, the automatic tonies.json update), every picture that is not cached yet,
  i.e. with a fresh cache all of them.

The reload request then does not answer for many minutes, and cache_preload = false has no effect.

To Reproduce

1. Empty cache folder, tonie_json.cache_images = true, tonie_json.cache_preload = false
2. Start teddyCloud, then call /api/toniesJsonReload
3. The request hangs; the cache folder fills with one picture after the other

Expected behavior
Adding a picture to the cache only creates its entry, as in 0.7.0. Pictures are downloaded during the load only with
tonie_json.cache_preload.

Technical Details:

• Version: develop (907acea); 0.7.0 is not affected

Additional context
A fix follows as PR.