Allow multiple simultaneous connections to the same peer

[tomaka]

Right now, in order to simplify the implementations of NetworkBehaviour, we only have allow connection per peer ID.
Allowing multiple connections would, in my opinion, considerably increase the difficulty of implementing this trait. If we provide helpers for NetworkBehaviour, though, we could allow multiple connections per peer.

Note that we should have ways to properly identify each connection. Having multiple connections shouldn't be implicit.

[tomaka]

Allowing multiple connections would, in my opinion, considerably increase the difficulty of implementing this trait. If we provide helpers for NetworkBehaviour, though, we could allow multiple connections per peer.

I just realized that is not necessarily true. We can tell the behaviour that "connected" means "connected for the first time", and "disconnected" means "zero connections remaining".
There could be either one NodeHandler for all the connections, or one NodeHandler per connection, I'm not sure.

[tomaka]

Bumping to "important". See paritytech/substrate#4272 for context.

[romanb]

This is now in libp2p-core, but libp2p-swarm does not yet permit explicitly creating multiple connections per peer. I guess this ticket should stay open until libp2p-swarm fully "embraces" the notion of multiple connections per peer?

[romanb]

Considered closed with #1519.