Fix a bug where resources pools could empty with waiters

knots/resource-pool.scm

@@ -939,6 +939,10 @@
                               (eq? x resource))
                             resources)))
 
+           (when (and (not (q-empty? waiters))
+                      (< (- (length resources) 1)
+                         max-size))
+             (spawn-fiber-to-return-new-resource))
 
            (loop (if index
                      (remove-at-index! resources index)