Background revalidation should be triggered after sending to the client about ledge

[hamishforbes]

Is there any point in using the ngx.thread method if we're just going to wait until we're done with the normal request processing before firing a background request anyway?

[pintsized]

No, probably not. Well, unless the thread protects the connection from closing prematurely in any way? Otherwise I think it's just a code sequence refactor.

[hamishforbes]

Even if we move the background revalidate request to after the ngx.eof() call with ESI there is the issue of what to do with sub-requests.
If they are also being served stale and revalidated in the background then that will block the main request until the ESI fragment has revalidated in the 'background', even though its returned stale content.
We can use ngx.is_subrequest var to prevent background revalidating sub-requests as this is never going to be a useful thing to do unless its truly run in the background.

Additionally should we be passing on the sub-request's Warning header when it is served stale but the main request is not? That would seem like the right thing to do.