unix: fix req cb / close cb invoke delay

Finalization of closed handles and associated requests was sometimes delayed
until an external event (network I/O, timeout, etc.) happened. This commit
addresses that.

src/unix/core.c

@@ -233,6 +233,9 @@ static unsigned int uv__poll_timeout(uv_loop_t* loop) {
   if (!ngx_queue_empty(&loop->idle_handles))
     return 0;
 
+  if (loop->closing_handles)
+    return 0;
+
   return uv__next_timeout(loop);
 }