MEDIUM: config: report a warning when multiple servers have the same name

A config where multiple servers have the same name in the same backend is
prone to a number of issues : logs are not really exploitable, stats get
really tricky and even harder to change, etc...

In fact, it can be safe to have the same name between multiple servers only
when their respective IDs are known and used. So now we detect this situation
and emit a warning for the first conflict detected per server if any of the
servers uses an automatic ID.