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

MEDIUM: stats: convert stats_dump_li_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 is worth mentionning that l->cum_conn is being dumped into a cum_sess
field and that once we introduce an official cum_conn field we may have
to dump the same value at both places to maintain compatibility with the
existing stats.