The c->sent is reset to 0 on each request by server-side http code,
so do the same on client side. This allows to count number of bytes
sent in a particular request.
The original idea was to optimize edge cases in case of interchangeable
backends, i.e. don't establish a new connection if we have any one
cached. This causes more harm than good though, as it screws up
underlying balancer's idea about backends used and may result in
various unexpected problems.
