Fix #277 Add connections on inbound packets

[ncallaway]

This is a proposed fix for #277.

I've confirmed that the behavior of the heartbeat demo seems more stable after this change:

server

$ cargo run -- -s                                                                                                 
[171.1µs] Bound to 127.0.0.1:12350
        [9.39ms] Connection from: 127.0.0.1:12355
[17.26ms] Received "Ping" from V4(127.0.0.1)

client

$ cargo run -- -c
[153.3µs] Bound to 127.0.0.1:12355
        [272.50µs] Sent: Ping
        [5.00s] Timeout from: 127.0.0.1:12350

I'm somewhat wary of this change due to the comment from the original refactor (#258). I'd love to have @TimonPost or @fraillt give critical thought and feedback about whether this approach might cause any issues.

[fraillt]

As I mentioned in #277 the main idea was to accept only by responding from the server. I think it is a security thing, as it allows server to select connections that it wants to accept, instead of someone else connecting without permission.

[ncallaway]

I'm closing this based on @fraillt's comments and the discussion in #277.

The longer term resolution for this will be implementing a proper handshake. The shorter term resolution will likely be documentation updates.