Make addTakeoverListener() part of the top-level server API, instead of
being specific to LibEventServerWithTakeover. Servers that don't
support socket takeover simply ignore this call.
This will allow future server implementations to support socket
takeover. This also allows us to start making the server creation logic
in HttpServer a bit more generic, since the code can always call
addTakeoverListener() without having to know if it really has a
LibEventServerWithTakeover or some other type of server.
Adam Simpkins
b69fe9baf1