Unreliable async thumbnails generation

Now that the picture thumbnails are produced by an async worker, I can quickly get to a situation in which thumbnails can no longer be generated: just upload large amounts of pictures at the same time (like ~50*500ko pictures through drag'n'drop), and at some point no more thumbnails will be generated, no matter how long you wait.

I'm attaching screnshots of a scenario with Nuxeo DAM, but the same problem also occurs with DM.

Some additional info:

• The issue is the same with EXIF/IPTC metadata, it is missing on pictures without thumbs
• Restarting Nuxeo or deleting completely all Pictures from Nuxeo usually helps the workers work again ("usually", because while I can't seem to get anymore thumbnails at all on certain servers, these workarounds always worked on my vanilla Nuxeo 5.7.1)