Move bootstrap logic to DHT

[Stebalien]

Really, each service should maintain a list of bootstrap peers for that service. The point of bootstrapping isn't to connect to some random nodes, it's to join the DHT.

Motivation: more and more peers are using the DHT in client mode. That means we need to re-bootstrap even if we have connections because we need to maintain at least one connection to at least one DHT node.

P0 because my node is falling off the network.

[Stebalien]

See: libp2p/go-libp2p-kad-dht#193

[meyer9]

It seems like we should even have a minimum number of peers per protocol. For example, we might want to connect to at least one node providing a specific service.

[Stebalien]

Status: libp2p/go-libp2p-kad-dht#387, libp2p/go-libp2p-kad-dht#383.

[Stebalien]

@meyer9 that really depends on the service, but not usually.

• Bitswap: we want to connect to peers with the content we're looking for. We use the DHT to find these peers.
• Pubsub: we want to connect to peers interested in the same topics. Again, we use the DHT.
• Relay: we need some relay node that's willing to act as a relay. We don't want a random node that speaks the relay protocol. We currently use the DHT to find these nodes.
• AutoNAT: Anyone who speaks the AutoNAT protocol is fine. We passively discover these nodes (e.g., our bootstrap nodes are "AutoNAT" nodes).
• DHT: we need to connect to some peer. In this case, we need a set of known DHT nodes.