Attempt to concurrently connect to remote IP addresses

[ClearlyClaire]

Currently, Mastodon tries to connect one IP at a time, which means it will waste time if the first IP address it tries is unreachable.

Furthermore, it should address the following scenario:

• instance A has two IP addresses
• the first one is unreachable to instance B, the second is reachable to instance B
• A issues a request to B, who needs to fetch a resource at A before replying
• B timeouts on the first address, and thus can only reply after the timeout duration
• which means A timeouts before allowing B to reply

[ClearlyClaire]

I am not super happy about having a socket as a hash key, but I'm not sure how else to make the sockaddr corresponding to the socket when doing the IO.select bit. Should be fine in practice.