Glance Image Cache Invalid Cache Entry and Stalled Image cleaner
This is meant to be run as a periodic task from cron.
If something goes wrong while we’re caching an image (for example the fetch times out, or an exception is raised), we create an ‘invalid’ entry. These entries are left around for debugging purposes. However, after some period of time, we want to clean these up.
Also, if an incomplete image hangs around past the image_cache_stall_time period, we automatically sweep it up.