fix(runloop): skip router rebuilding if reconfigure is running (#14056)

In rare cases, the rebuild timer will interrupt the reconfigure handler because of `router was changed while rebuilding it`, which results in an outdated plugin iterator and balancer.

It is hard to test this rare race condition.

KAG-6015

kong/runloop/handler.lua

@@ -992,8 +992,30 @@ return {
       if strategy ~= "off" or kong.sync then
         local worker_state_update_frequency = kong.configuration.worker_state_update_frequency or 1
 
+        --[[
+                    +-----------+
+                    |   Start   | <-------------------------------------+
+                    +-----------+                                       |
+                          |                                             |
+                          |                                             |
+                          v                                             |
+                    ***************************           +-------+     |
+                    * Is reconfigure running? * ---Yes--->| Sleep | ----+
+                    ***************************           +-------+
+                          |                                   ^
+                          No                                  |
+                          |                                   |
+                          v                                   |
+                    +---------------+                         |
+                    | rebuild router|-------------------------+
+                    +---------------+
+
+            Since reconfigure will also rebuild the router, we skip this round
+            of rebuilding the router.
+        --]]
         local router_async_opts = {
-          name = "router",
+          name = RECONFIGURE_OPTS and RECONFIGURE_OPTS.name or "router", -- please check the above diagram for the
+                                        -- reason of using the same name as reconfigure
           timeout = 0,
           on_timeout = "return_true",
         }