Implement IPv6 support for distributed XGBoost

Currently distributed XGBoost does not support IPv6, this is explicitly asserted in rabit/include/rabit/internal/socket.h
Our internal network is IPv6 only, which means we can't run XGBoost out of the box.

IPv6 is 20+ years old, and IPv4 is being phased out more and more as the time progresses. I think it is very important to enable IPv6 in XGBoost for more successful adoption.