Fix to work with new iron api

This uses the request extensions to store the start time in the request. I played around w/ implementing it as an `AroundMiddleware`, but it seems you'd always want this to be the first before and the last after, hence it's a pair of middlewares.
Can there instead be a private key type, perhaps `StartTime` or something along those lines? If you don't export the type, there is no chance of other middleware incorrectly using it.