forward proxy should be able to be configured to prefer IPv6 over IPv4 for outbond connections

When the forward proxy cache server is connected to a dual stack network which both IPv4 and IPv6 are available, it should be able to be configured to prefer IPv6 over IPv4 for outbond connections, as it's now a common behavior of various software including all mainstream web browsers.

I didn't check the code but now there seems to be no such a switch to tweak the preferred protocol, and it seems it prefers IPv4 over IPv6 (or maybe it's because DNS resolving is faster for IPv4?).