MEDIUM: stats: convert stats_dump_be_stats() to use stats_dump_fields_csv()

MEDIUM: stats: convert stats_dump_be_stats() to use stats_dump_fields_csv()

This function now only fills the relevant fields with raw values and
calls stats_dump_fields_csv() for the CSV part. The output remains
exactly the same for now. It's worth noting that there are some
ambiguities between connections and sessions, for example cum_conn
is dumped into cum_sess. Additionally, there is a naming ambiguity
in that the internal "d_time" (time where the beginning of data
appeared) is called "rtime" in the output (response time) and they
actually are indeed the same.