Improve BFD settings

[morimoto-cybozu]

What

We observed that a Dell 15G server held network packets for about 1 second when its iDRAC had been reset.
This delay of network packets caused the server's network links to get down for about 50 seconds.
This was because the BIRD Internet Routing Daemon for the network links was configured with 500 milliseconds BFD timeout as follows.

• min rx interval 100 ms
• min tx interval 100 ms
• default multiplier of 5

cf. https://github.com/cybozu-go/neco/blob/release-2022.08.16-19406/installer/setup/templates.go#L15
cf. https://github.com/cybozu-go/neco/blob/release-2022.08.16-19406/ignitions/common/files/etc/bird/bird.conf#L9

Note that the settings of the network switches would also relate.

We should:

1. improve the BFD settings so that 1 second delay does not cause the loss of network connection.
2. investigate why the short delay causes so long disconnection, and improve the settings.

How

Describe how to address the issue.

Checklist

• Finish implementation of the issue
• Test all functions
• Have enough logs to trace activities
• Notify developers of necessary actions