fix(queues): shutdown flag needs to be checked while waiting (#11456)

The previous fix (#11376) to enable batching during shutdown failed to check
the shutdown flag while waiting for more items to batch.  If the coalescing
delay was large enough, this could cause the last batch to be lost.

(cherry picked from commit 3bf77d7)

kong/tools/queue.lua

@@ -242,6 +242,12 @@ function Queue:process_once()
     -- so that we can check for worker shutdown periodically.
     local wait_time = math_min(self.max_coalescing_delay - (now() - data_started), COALESCE_POLL_TIME)
 
+    if ngx.worker.exiting() then
+      -- minimize coalescing delay during shutdown to quickly process remaining entries
+      self.max_coalescing_delay = COALESCE_MIN_TIME
+      wait_time = COALESCE_MIN_TIME
+    end
+
     ok, err = self.semaphore:wait(wait_time)
     if not ok and err ~= "timeout" then
       self:log_err("could not wait for semaphore: %s", err)