Implement RFC 8305, the "Happy Eyeballs".

[midnight-wonderer]

Overview

This addresses #739.

Happy Eyeballs landed in Ruby for quite some time. 🎉
https://bugs.ruby-lang.org/issues/20108

However, it does not reach HTTP.rb yet.
This is because HTTP.rb uses TCPSocket.open by default.

Source traces

• open was implemented in the IO class as an alias to new.
  https://github.com/ruby/ruby/blob/37a16c7812f5b7e6faa762b927e9f04065cc495a/io.c#L8124
• In the case of TCPSocket, it is bound directly to the c source code.
  https://github.com/ruby/ruby/blob/6deeec5d459ecff5ec4628523b14ac7379fd942e/ext/socket/tcpsocket.c#L54
• But Happy Eyeballs was implemented in the Ruby layer.
  ruby/ruby@9ec342e

A baby step

In order to take advantage of the new feature. I alias default_socket_class to Socket.tcp so the connection logic will be handled by the Ruby layer of Socket.

Rough edges

For your information Socket.tcp has a lot of features beyond Happy Eyeballs.
For example, it supports connect_timeout natively.
What to do with

http/lib/http/timeout/global.rb

Line 24 in db6863a

::Timeout.timeout(@time_left, ConnectTimeoutError) do

needs more discussions.
This is just one aspect of it.

Nevertheless, this baby step would introduce minimal breaking changes. (or any changes at all)
Providing more robust connections.