Deleting the last NatsJetStreamChannel in a namespace when namespace-scoped can result in orphaned resources

[dan-j]

Describe the bug
If you have a NatsJetStreamChannel in a namespace with the eventing.knative.dev/scope: namespace annotation set, a dispatcher is deployed to that namespace. If the channel is then deleted, the controller deletes the dispatcher deployment, but sometimes the dispatcher doesn't have time to delete the underlying stream and/or consumers.

We can't move the stream/consumer management to the controller, because the credentials to the NATS cluster are based on the dispatcher (and we have different credentials in different namespaces).

What we really need to do is have multiple finalizers on the channel, so the actual channel isn't deleted until both reconcilers have time to finalize the resource. Once all finalizers are removed, it will then be deleted and the controller can remove the dispatcher deployment.

Expected behavior
Any underlying resources are cleaned up when a channel is deleted.

To Reproduce
See above.

Knative release version
Latest

Additional context
I already have the start of this fix in a big WIP which includes #542 and #543