core: make queue_state_fiber cancellable

We are going to finish all client (non-system) fibers in the process of
Tarantool shutdown. First we cancel them and then wait for their
finishing. So if the client fibers are not finished Tarantool shutdown
is never finished too.

Currently the fiber can not be finished and we got hang in integration
testing of PR tarantool/tarantool#9604.

Author: nshy

Diff for: queue/abstract/queue_state.lua

@@ -85,8 +85,12 @@ local function create_state_fiber(on_state_change_cb)
                     log.info('Queue state changed: WAITING')
                 end
             end
+            -- Handle server shutdown.
+            if not pcall(fiber.testcancel) then break end
         end
     end)
+
+    log.info('Finished queue state fiber')
 end
 
 -- Public methods.