Smarter handling of Last-Modified

[wezm]

Currently (in the Rust version) the Last-Modified header is set to the mtime of the document. A failure mode here is if a new version of the application is deployed that alters the way pages are rendered requests with If-Modified-Since will still be considered fresh. Seems like Last-Modified should be set to max(page_mtime, build_date) where build date is a timestamp set when the binary was compiled.