Relay: automatically tie the served device's second channel to its serving device's second channel

[StevenCellist]

Summary

Instead of requiring to configure the served device's second channel manually, automatically link it to its serving device (relay).

Current Situation

The user is required to manually configure the Frequency, Datarate and Offset fields for the served device as well as the serving device.

Why do we need this? Who uses it, and when?

Tying the second channel for the served device to the serving device's configuration means that the user cannot mistakenly misconfigure the device. I also so no single reason why the served device would be configurated differently to the serving device (relay), as there would be no way for the relay to catch WOR signals on a misconfigured channel.

Proposed Implementation

Remove the --mode.served.second-channel options and instead automatically apply these settings from the --mode.serving.second-channel.

Contributing

• [X] I can help by doing more research.
• [ ] I can help by implementing the feature after the proposal above is approved.
• [X] I can help by testing the feature before it's released.

Validation

• [ ] The feature is tested in a staging environment.
• [ ] The feature is documented in The Things Stack Documentation

Code of Conduct

• [X] I agree to follow TTN's Community Code of Conduct.