MINOR: cli: add more information to the "show info" output

In addition to previous outputs, we also emit the cumulated number of
connections, the cumulated number of requests, the maximum allowed
SSL connection concurrency, the current number of SSL connections and
the cumulated number of SSL connections. This will help troubleshoot
systems which experience memory shortage due to SSL.
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".
MEDIUM: listener: apply a limit on the session rate submitted to SSL

Just like the previous commit, we sometimes want to limit the rate of
incoming SSL connections. While it can be done for a frontend, it was
not possible for a whole process, which makes sense when multiple
processes are running on a system to server multiple customers.

The new global "maxsslrate" setting is usable to fix a limit on the
session rate going to the SSL frontends. The limits applies before
the SSL handshake and not after, so that it saves the SSL stack from
expensive key computations that would finally be aborted before being
accounted for.

The same setting may be changed at run time on the CLI using
"set rate-limit ssl-session global".
REORG: stats: move the stats socket states to dumpstats.c

There is no more usage of these values outside of dumpstats.c, and
they're easier to maintain there. Also replace the #defines with an
enum.
MINOR: cli: add the new "show pools" command

show pools
  Dump the status of internal memory pools. This is useful to track memory
  usage when suspecting a memory leak for example. It does exactly the same
  as the SIGQUIT when running in foreground except that it does not flush
  the pools.