Size-related VCL variables

To allow better reporting, Fastly has added several variables to VCL to give more insight into what happened in a request.

Name	Where	Description
`req.bytes_read`	deliver log	How big a request was in total bytes.
`req.header_bytes_read`	all	How big the header of a request was in total bytes.
`req.body_bytes_read`	deliver log	How big the body of a request was in total bytes.
`resp.bytes_written`	log	How many bytes in total were sent as a response.
`resp.header_bytes_written`	log	How many bytes were written for the header of a response.
`resp.body_bytes_written`	log	How many bytes were written for body of a response.
`resp.completed`	log	Whether the response completed successfully or not.