Support random dropping of expensive rewrites

[GoogleCodeExporter]

Image rewrites are CPU intensive.  For some large sites with many images, some 
images are frequently fetched while the majority of images are infrequently 
fetched.  Optimizing every image can be wasteful for images that are unlikely 
to be fetched again soon.  

Keeping track of frequency counts for every image is a memory and complexity 
burden.  Instead, we should support random drops of image rewrites, which is 
stateless.  For instance, if 90% of image rewrites are dropped, then there is a 
10% chance of optimizing an image fetched only once, but a 99% chance of 
optimizing an image fetched 50 times. 

Original issue reported on code.google.com by jkar...@google.com on 5 Jun 2013 at 1:19