Don't use singleton in routes.middleware.RoutesMiddleware

It seems that using default singleton=True in the
routes.middleware.RoutesMiddleware which is leading to use thread-local
RequestConfig singleton object is not working well with eventlet
monkeypatching of threading library which we are doing in Neutron.
As a result it leaks memory in neutron-api workers every time when API
request to not existing API endpoint is made by user.

To avoid that memory leak, let's use singletone=False in that
RoutesMiddleware object, at least until problem with thread-local
singleton and eventlet monkey patching will be solved.

Closes-Bug: #1942179
Change-Id: Id3a529248d3984506f0166bdc32e334127a01b7b

neutron/api/extensions.py

@@ -200,8 +200,13 @@ def __init__(self, application,
             controller = req_controllers[request_ext.key]
             controller.add_handler(request_ext.handler)
 
+        # NOTE(slaweq): It seems that using singleton=True in conjunction
+        # with eventlet monkey patching of the threading library doesn't work
+        # well and there is memory leak. See
+        # https://bugs.launchpad.net/neutron/+bug/1942179 for details
         self._router = routes.middleware.RoutesMiddleware(self._dispatch,
-                                                          mapper)
+                                                          mapper,
+                                                          singleton=False)
         super(ExtensionMiddleware, self).__init__(application)
 
     @classmethod