Retain reference in Lwt_react.S.with_finaliser
Retain reference in Lwt_react.S.with_finaliser

This is a counterpart to 608e9cc36e3c7847f77d828aecb0f9d084532834, and
part of #626.

ocsigen/lwt,ocsigen/lwt,c-cube/lwt,c-cube/lwt,c-cube/lwt,ocsigen/lwt