Add IPv4-only HTTP client

[robertodauria]

This replaces the default HTTP client from the http package with a client that will always connect over IPv4.

The main reason behind this change is that the Autojoin API currently tries to autodetect the node's IPv4 by reading the HTTP request's client IP. On dual-stack machines, the request could randomly happen over IPv6 and the Autojoin API wouldn't have the minimum information required to register the node. This change mitigates that problem.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 10510110774

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.212%

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Totals
Change from base Build 10495869920:	0.0%
Covered Lines:	714
Relevant Lines:	727

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💛 - Coveralls

[stephen-soltesz]

Would it be preferable overall to make ipv4= parameter required? If we eliminated the fallback in the server then clients would have to specify the address at registration time, and the http connection could use v4 or v6.

Reviewable status: 0 of 1 approvals obtained

[robertodauria]

After discussing this on Slack, do you think it's worth keeping the ipv4 discovery?

Reviewable status: 0 of 1 approvals obtained

[stephen-soltesz]

Yes, I think so. Now I'm wondering if it's a mistake to have the ipv4= parameter! :)

Reviewable status: complete! 1 of 1 approvals obtained

[robertodauria]

It would help in handling the case where the server has multiple interfaces/addresses, and the host wants to serve ndt-server on a specific one (which may not be the default route).