bevy_matchbox freezes infinitely if you use `MultipleChannels` plurality

[simbleau]

Hard to make a reproducible example, but I am working on it.

Here's what I've done:

• Change the bevy_ggrs example to use try_update_peers instead of update_peers (which would prevent the example from panicking when it fails to connect to the signaling server)
• Change the example to use bevy_matchbox... Change to use commands.open_socket() and .close_socket()

And finally, the part that will hang the program indefinitely:

• Add an additional channel.

If it uses a single channel it will work as intended.

[simbleau]

The PR above updates the bevy-ggrs example to use bevy_matchbox.

It is blocked on #283 unfortunately.

[simbleau]

I isolated the issue and found out the issue.

socket.channel(RELIABLE_CHANNEL_INDEX).receive() can hang infinitely.

This is talked about and fixed in #329