Switched to using posted next events after sendfile_max_chunk.

Previously, 1 millisecond delay was used instead.  In certain edge cases
this might result in noticeable performance degradation though, notably on
Linux with typical CONFIG_HZ=250 (so 1ms delay becomes 4ms),
sendfile_max_chunk 2m, and link speed above 2.5 Gbps.

Using posted next events removes the artificial delay and makes processing
fast in all cases.

src/http/ngx_http_write_filter_module.c

@@ -331,8 +331,7 @@ ngx_http_write_filter(ngx_http_request_t *r, ngx_chain_t *in)
         && c->write->ready
         && c->sent - sent >= limit - (off_t) (2 * ngx_pagesize))
     {
-        c->write->delayed = 1;
-        ngx_add_timer(c->write, 1);
+        ngx_post_event(c->write, &ngx_posted_next_events);
     }
 
     for (cl = r->out; cl && cl != chain; /* void */) {