engine: net.ParseIP does not understand IPv6 scope ID

[bassosimone]

Code inside the engine uses net.ParseIP to parse resolved IP addresses. Go stdlib resolver (and ooni/probe-cli#698) consider the IPv6 scope ID when serializing IP addresses. And net.ParseIP does not handle the scope. Therefore, if a DNS resolution returns a scoped IP address, we fail to parse it. In the specific context of ooni/probe-cli#698, the symptom was that sessionresolver failed with dns_bogon_error because it could not parse the returned IPv6 addresses and thus concluded it was passed a bogon (this is an unfortunate issue of the design of the code for detecting bogons). After noticing that, I changed the code to not include the scope ID if it is zero. But there is still a ticking bomb that basically most of the engine is not prepared to serialized scoped IPv6 addresses (so, e.g., a DHCP server may give out scoped IPv6 addresses representing the default resolver). I think in most cases this will not be a problem, but we know that IPv6 uses the scope for link-local addresses. We'll consider this issue done when we'll have figured out a better and scope-aware way of parsing IP addresses and we'll have removed net.ParseIP from our code base.