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:

janus-ftl-plugin/src/JanusFtl.cpp

Line 765 in 9315b8b

if (streams.count(channelId) <= 0)

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