MEDIUM: snapshot: start to reorder the HTTP snapshot output a little bit

MEDIUM: snapshot: start to reorder the HTTP snapshot output a little bit

The output of "show errors" was slightly reordered to split the HTTP part
in a single chunk_appendf() call. The useless buffer total input was
replaced to report the buffer's start offset, which is the offset in the
stream of the first input byte (thus not counting output). Also it was
the opportunity to stop calling the stream "session".