Channel global decoupling for region discovery

Ably is a platform for pub/sub messaging that operates across multiple regions. The original design involved a GlobalChannelResource responsible for determining active regions and communicating updates, but it had issues with activation speed, resource usage, and reliability in the face of regional disruptions. To address these problems, Ably implemented a new region discovery mechanism called 'channel global decoupling.' This change involves peer-to-peer region discovery without relying on a global role. The new design has significantly improved channel activation times, reduced resource usage, and increased reliability during regional disruptions.