Edge nodes should keep track of orchestrator channel subscriptions instead of relying on active streams

[haydenmc]

Stale FtlClient relay connections may still exist even after a channel subscription is removed, causing the Edge node to potentially send extra unsubscribe events.

See here: https://github.com/Glimesh/janus-ftl-plugin/blob/9315b8b045f584e3cfc7e4c65ac761275c679947/src/JanusFtl.cpp#L765

Solution: Edge needs to keep track of its subscriptions, and not just unsubscribe/subscribe based on active streams