Consider file modification time for tile and info cache

[jechols]

When a file is replaced by another with different dimensions, the cache is a mess and wrecks things until the cache(s) expire. Since we don't expire based on a time, instead opting for a "most popular" kind of cache, the expiration can be seconds, hours, days, weeks, etc.

It might take a bit of work to figure this out, but it would really be ideal to have cached requests actually look at the file just to get a quick stat - if the mod time is more recent than the cached data, don't serve up the cached data.

Of course we don't currently store any kind of cached data timestamp, so that would be the first step to having a smarter cache.