http2

[bfarber]

We previously pushed the headers as mentioned, however we were running into cases where we'd send massive header lines because the client needed 25 javascript files and 25 CSS files, and this would cause 5XX server errors (due to the header size being too large). Further, we were getting some reports that the push headers were causing more harm than good because it forced the browser to fetch those files right away even when sometimes they weren't needed right away. Anyways, as noted, we stopped sending the push preload header until a better methodology of determining the most important assets to push could be worked out.