Server: further streamline caching structure.

This allows for a much better experience when working on a slow link.

- Allow caching of everything (including perl module status and HTML
  pages) for 4 hours, by default.
- Provide an option to change the cache lifetime (-c=0 is more or less
  equivalent to -f)
- Store un-changed files in cache, updating them just before preparing
  to serve.
- Always read from cached file, which helps ensure they will be
  reusable, even when the force is on.

Still a problem: country.json, possibly Spice script-loading order.