[PATCH] log received body size

Added %http::<bs and %icap::<bs logformat codes to HTTP and ICAP body
sizes received from the next HTTP hop or the ICAP server.

Logging "received message body" is useful because Squid may receive a
lot more or a lot less than it serves to the client or than the original
resource size, which may happen when handling Range requests and partial
responses, when adapting bodies, and for other reasons.

For HTTP, we define "received message body" as message body bytes that
Squid stores, merges, adapts, and/or forwards. In most cases, they are
the same as body bytes sent by the server to Squid. However, the two
bodies may differ for reasons such as errors (where the start of the
body was not found), HTTP transfer encodings (where Squid strips chunked
encoding to find the message body), and generated FTP directory listings
(that were received in a completely different format on a control
connection).

For ICAP, the "received message body" is the Encapsulated sections,
after the encapsulated HTTP body, if any, is dechunked.

This is a Measurement Factory project.

Regards,
     Christos