[optimizer] allow http caching for bundles (elastic#15880)

By setting cache-control to must-revalidate, we indicate to the browser
that it should send the if-none-match header with the last known etag
value. If the etags match, the server responds with a 304 and no body,
and if they don't the server responds with a 200 and a body along with a
new etag.

This ensures a cache is always invalidated when a bundle changes, like
when Kibana is upgraded or a plugin is installed.

We no longer send the last-modified header because we don't want the
browser caching based on time. Doing so can be unpredictable since the
browser may not agree with the server's definitions of DST, and the
server time itself can be changed.

src/optimize/bundles_route/dynamic_asset_response.js

@@ -67,7 +67,7 @@ export async function createDynamicAssetResponse(options) {
     const response = request.generateResponse(replacePlaceholder(read, publicPath));
     response.code(200);
     response.etag(`${hash}-${publicPath}`);
-    response.header('last-modified', stat.mtime.toUTCString());
+    response.header('cache-control', 'must-revalidate');
     response.type(request.server.mime.path(path).type);
     return response;