Fix a bug with resource pool idle checking

The fiber would never finish.

knots/resource-pool.scm

@@ -1178,9 +1178,16 @@
      (when idle-seconds
        (spawn-fiber
         (lambda ()
-          (while #t
-            (sleep idle-seconds)
-            (put-message channel '(check-for-idle-resources))))))
+          (let loop ()
+            (put-message channel '(check-for-idle-resources))
+            (when (choice-operation
+                   (wrap-operation
+                    (sleep-operation idle-seconds)
+                    (const #t))
+                   (wrap-operation
+                    (wait-operation destroy-condition)
+                    (const #f)))
+              (loop))))))
 
      (with-exception-handler
          (lambda (exn)