Fix requestLog (CU_REQUEST_LOG)

Note that this is currently disabled in production since it broke when we
moved to express 4, since it removes app.routes.

`req.route` is populated after next() is called, so we can use it here.

I'd like to add that it is slightly nasty that this is a thing which isn't
exercised during testing (because it's only used when CU_REQUEST_LOG is set),
and therefore meant we didn't catch this before we tried to go live. Blegh.