(This is not fully general yet: we only pick randomly among
_supported_ addresses, and each extendinfo contains at most one IPv4
address and at most one IPv6 address, no matter what the extend cell
had.)
This change will help dual-stack relays do IPv6 reachability tests,
in theory, by having them sometimes do IPv4 connections and
sometimes do ipv6 connections.
Closes ticket 33220.
