CLEANUP: counters: move from 3 types to 2 types

We used to have 3 types of counters with a huge overlap :
  - listener counters : stats collected for each bind line
  - proxy counters : union of the frontend and backend counters
  - server counters : stats collected per server

It happens that quite a good part was common between listeners and
proxies due to the frontend counters being updated at the two locations,
and that similarly the server and proxy counters were overlapping and
being updated together.

This patch cleans this up to propose only two types of counters :
  - fe_counters: used by frontends and listeners, related to
    incoming connections activity
  - be_counters: used by backends and servers, related to outgoing
    connections activity

This allowed to remove some non-sensical counters from both parts. For
frontends, the following entries were removed :

  cum_lbconn, last_sess, nbpend_max, failed_conns, failed_resp,
  retries, redispatches, q_time, c_time, d_time, t_time

For backends, this ones was removed : intercepted_req.

While doing this it was discovered that we used to incorrectly report
intercepted_req for backends in the HTML stats, which was always zero
since it's never updated.

Also it revealed a few inconsistencies (which were not fixed as they
are harmless). For example, backends count connections (cum_conn)
instead of sessions while servers count sessions and not connections.

Over the long term, some extra cleanups may be performed by having
some counters update functions touching both the server and backend
at the same time, as well as both the frontend and listener, to
ensure that all sides have all their stats properly filled. The stats
dump will also be able to factor the dump functions by counter types.
6 files changed