MINOR: stats: add 3 fields to report the frontend-specific connection stats

MINOR: stats: add 3 fields to report the frontend-specific connection stats

Frontends have extra information compared to other entities, they can
report some statistics at the connection level while the other ones
are limited to the session level. This patch adds 3 more fields for
this :
 - conn_rate
 - conn_rate_max
 - conn_tot

It's worth noting that listeners theorically have such statistics, except
that the distinction between connections and sessions is not clearly made
in the code, so that will have to be improved later.