Option to cache results of uri validation functions

Wouldn't be too hard to implement a cache table to store which URIs are broken and which are working fine so we aren't pinging our file system for info on every single image.

Whether getting this information from the cache or by checking the file system directly for each file is actually faster or not in practice is another matter entirely...

Drupal's db cache kinda sucks and is probably slower or about the same speed as just working with the files, but anyone using something "advanced" like memcache that pushes the cache into memory would probably appreciate this feature. Because of this, any cache implementation would have to be enabled through an extra, optional admin setting.

This ticket is related to the discussion here #908282: Remove unnecessary I/O from theme_image().