RouterInfo: ensure return of ipv6-capable Address object when requested

[anonimal]

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By submitting this issue, I confirm the following:

• I have read and understood the contributor guide in kovri-docs.
• I have checked that the issue I am reporting can be replicated or that the feature I am suggesting is not present.
• I have checked opened or recently closed pull requests for existing solutions/implementations to my issue/suggestion.

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As noted by my hasty typo, this should've been a TODO:

• An RI should always have an IPv4 address. Checks for that shouldn't be done here: assertions should / should eventually be in place for that elsewhere.
• The check assumes that the first iteration of Addresses will be ipv6. It's not safe to assume.

Referencing #627.

[anonimal]

@brbzull0 is working on this / in this area (directly / indirectly with #831).

[anonimal]

He ended up not working on this.

In my recent work on this issue, I've found that fixing this actually uncovers pre-forked i2pd issues. So, fixing one issue will actually create more 😄 This is fine though, but the resolution of all issues just may not make it into this upcoming release. On a better note, the fixes also appear to resolve #187. This work is in a local branch but I can push sometime next week.

[anonimal]

Note: #984 decided to side-step most of the pre-fork issues mentioned in #840 (comment) in favor of resolving a few immediate ones (including the adjusting of appropriate SSU packet size).

[anonimal]

Referencing #140.