Support cache revalidation headers (ETag/Last-Updated)

[lann]

Support either (or both) of:

• etag/if-none-match: Server sends etag header that must change if the file changes (e.g. hash of the file contents). If a client sends if-none-match with a matching value, server may return empty 204 Not Modified. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match
• last-modified/if-modified-since: Server sends last-modified with file modification timestamp. If a client sends if-modified-since >= modification timestamp server may return empty 204 Not Modified. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since