MEDIUM: listener: add support for limiting the session rate in addition to the connec...

MEDIUM: listener: add support for limiting the session rate in addition to the connection rate

It's sometimes useful to be able to limit the connection rate on a machine
running many haproxy instances (eg: per customer) but it removes the ability
for that machine to defend itself against a DoS. Thus, better also provide a
limit on the session rate, which does not include the connections rejected by
"tcp-request connection" rules. This permits to have much higher limits on
the connection rate without having to raise the session rate limit to insane
values.

The limit can be changed on the CLI using "set rate-limit sessions global",
or in the global section using "maxsessrate".