Fix high memory usage in retry middleware

integration/fixtures/retry/simple.toml

@@ -0,0 +1,25 @@
+defaultEntryPoints = ["http"]
+
+logLevel = "DEBUG"
+
+[entryPoints]
+  [entryPoints.http]
+  address = ":8000"
+
+[api]
+
+[retry]
+
+[file]
+[backends]
+  [backends.backend1]
+    [backends.backend1.servers.server1]
+    url = "http://{{.WhoamiEndpoint}}:8080" # not valid
+    [backends.backend1.servers.server2]
+    url = "http://{{.WhoamiEndpoint}}:80"
+
+[frontends]
+  [frontends.frontend1]
+  backend = "backend1"
+    [frontends.frontend1.routes.test_1]
+    rule = "PathPrefix:/"