Overhaul stats: Increase IPv4 stats for IPv6 IPs with the prefix ::ffff:

[josecelano]

Relates to: #1368

When you bind the server socket address to IPv6 IP addresses ([::]:6969 in the bind_address config option), the tracker can handle requests from IPv4 and IPv6 clients. IPv4 clients' addresses are converted to IPv6 IPs.

However, the metrics are increased for IPv6. I think we can add a config option to change the behaviour and increase IPv4-related metrics when the IPv6 address has the ::ffff: prefix.

I would make it the default behaviour, but that's a breaking change. So the new config option should be disabled.

IPv4-mapped notation

cc @da2ce7 @devnakx