Roadmap to non-experimental pubsub

[Stebalien]

The ipfs pubsub subsystem is currently an MVP and needs some hardening before it can truly be considered "ready":

1. Use pubsub's new built-in discovery system instead of literally putting a block advertising our interest in the topic into the blockstore.
2. Manage pubsub topics. This may involve changing the API. At the moment, calling pubsub publish will "join" the topic permanently.
3. Pass pubsub.WithReadyness(pubsub.MinTopicSize(1)) on publish so we don't publish into the void. We can pass a context with a short (1-10s) timeout to avoid blocking forever.
4. Use the non-deprecated topic.Publish function so we can actually pass in a context.

[Stebalien]

Missed #6621.