Node Joining Swarm

The PPSPP paper relies on a centralized tracker (PPSTP) or a DHT to discover the swarm.

For Livepeer, since there is already the underlining kademlia network, we would be able to route to the correct broadcaster (and treat the Livepeer kademlia network as the DHT).

For the stream-specific swarms, a simple approach would be for the broadcaster to keep track of all the peers in the swarm. We may still need to implement a tracker system later if the network isn't good at peer discovery, or if we need an incentivized tracker/seeder to fill in for deficiencies in the network.

Let's create something like `func join(strmID str)[]PeerInfo`, that way we can swap out the implementation if we decide to use trackers in the future.

Start by reading the existing Go PPSPP implementation and the surrounding peer-discovery or Kademlia integration, then trace how a stream-specific swarm is represented. Use the proposed join(strmID str)[]PeerInfo entry point to define the discovery boundary; done should include a clear node-joining path that can later accommodate tracker-based discovery.

Written by the indexing model from the issue text.