Fixed server errors on delete_quiz and increased thumbnail timeouts

Today's hotpatch (3b3bbbc) includes these changes:

Performance improvements

• Increased number of BlobMachine workers. Should allow fetching of more blobs concurrently.

Bugfixes

• It was not possible to remove your saved or sent answers because the delete_quiz request handler was generating an internal server error. This should now be fixed.
• Increased timeout for thumbnail and avatar request handlers to 30 seconds, to avoid timeout issues on large video objects.