Thumbnail rendition should not be processed asynchronously

NXP-31855 introduced an async thread pool for serving renditions with the @rendition rest api adapter. This is a welcome improvement for rendition that are computed on the fly.

However, for stored renditions with a very high call rate such as thumbnail, this is counterproductive:

• The URL can be returned immediately because the rendition is stored.
• If the pool is fully utilised by long renditions like document previews, thumbnails in webui won't load
• If the pool and queue is full of thumbnail renditions, it also impacts the time to serve document preview renditions.