Send x-associated-content header to enable server push

[ewinslow]

This is how we enable server push. That we haven't been doing this is just so sad...

Could dramatically improve initial load time by cutting down on round trips. Really useful for AMD modules which require the client to execute JavaScript before they're discovered, but everything else as well, honestly.

One thing I haven't checked on is whether clients or servers do anything special to detect whether the browser has perhaps already received the content, in which case pushing it would actually be a waste of bandwidth.

[ewinslow]

But frankly, I think the speed benefits are attractive enough to just do that first and then worry about how to optimize for bandwidth at the application level it we discover that clients/servers aren't already doing that.

[ewinslow]

So when browsers start getting a push for a resource that is already in cache, they will send a RESET response to cancel the transmission and conserve bandwidth. This seems like a perfectly good tradeoff to me.