[MEDIUM] stats: count server retries and redispatches

[MEDIUM] stats: count server retries and redispatches

It is important to know how your installation performs. Haproxy masks
connection errors, which is extremely good for a client but it is bad for
an administrator (except people believing that "ignorance is a bless").

Attached patch adds retries and redispatches counters, so now haproxy:

1. For server:
 - counts retried connections (masked or not)

2. For backends:
 - counts retried connections (masked or not) that happened to
    a slave server
 - counts redispatched connections
 - does not count successfully redispatched connections as backend errors.
    Errors are increased only when client does not get a valid response,
    in other words: with failed redispatch or when this function is not
    enabled.

3. For statistics:
 - display Retr (retries) and Redis (redispatches) as a "Warning"
   information.