core: refactor to remove modes

[tmigone]

This PR removes modes from balenaSound as a user facing setting and replaces them with a mechanism that automatically upgrades the device to "multi-room" when possible; this is how it works:

1. Devices boot into "standalone" mode, this means that multi-room is disabled initially and the device operates by itself routing audio from plugins directly into the output hardware.
2. When the sound-supervisor starts it will check if the device is capable of being upgraded into "multi-room" mode. This is basically a check against a device-type blacklist that prevents the upgrade on low end devices.
3. If the device can run as a "multi-room" server then the sound-supervisor will update the audio routes and restart multiroom-client so it picks up the multiroom server address.

Benefits
As a result of these changes the overall architecture is greatly simplified, most notably:

• The audio service no longer depends on sound-supervisor; audio routing is no longer dependent on which mode the device is on. This means in the event of a sound-supervisor startup failure the device will at least have the "standalone" fallback and will function adequately.
• The multiroom feature still depends on sound-supervisor but the services can startup regardless and won't be crash looping if it doesn't start:
  • multiroom-server no longer depends on sound-supervisor
  • multiroom-client service only relies on sound-supervisor to get the IP address of the multiroom master server, however it falls-back to localhost if the sound-supervisor is not available.

Removing the service inter-dependency is a huge win in terms of support as currently the vast majority of support threads are related to it. Currently the service dependency chain is: sound-supervisor > audio > multiroom-server > multiroom-client. Any one service failing to start will cause the application to fail.

To-do

• Update documentation
• Review env vars

Change-type: major
Signed-off-by: Tomás Migone [email protected]

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