Refactored sendfile() AIO preload. This reduces layering violation and simplifies the logic of AIO preread, since it's now triggered by the send chain function itself without falling back to the copy filter. The context of AIO operation is now stored per file buffer, which makes it possible to properly handle cases when multiple buffers come from different locations, each with its own configuration.

--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -574,9 +574,6 @@ struct ngx_http_request_s {
 
 typedef struct {
     ngx_http_posted_request_t         terminal_posted_request;
-#if (NGX_HAVE_AIO_SENDFILE)
-    u_char                            aio_preload;
-#endif
 } ngx_http_ephemeral_t;