Allow the fallback script to ignore temporary IPv6 addresses
When updateFallbackDirs.py checks relay addresses, it makes sure that the IPv4 and IPv6 addresses and ports match the relay's whitelist entry.
If a relay's IPv6 address is temporary, it should not be included in the whitelist.
But this means the relay will never be selected, because its descriptor has an IPv6 address, and that address doesn't match the (missing) address in the whitelist.
We should add a way to say ipv6=ignored or something.