Add a cleanup function when using add_routes

[wodadehencou]

I use per_req_config_modify to add callback handler, but I can not destroy the handler when the request is finished. So there are some memory leakage causing my application fail.

[eyurtsev]

Do you have example code to try to reproduce? A clean up function shouldn't be necessary -- this is not expected behavior and is either a bug in langserve or else in user code