Add custom logging for $customer
Also log full {,be}{req,resp} sizes
Initialise hdr_sz variables earlier
Print the bereq_sz field when there is no body too
Do more explicit tracking of request size
Move logging of what's written to the backend

FetchReqBody is called from cnt_hit as well as from FetchHdr, meaning
we would log bereq_sz for all hits.  Fix this by moving the code into
FetchHdr.
Log the body size correctly also when streaming
Use ssize_t for sizes
sp is NULL after fetchbody, cache a pointer