Previously, we'd consider "transiently" connected peers to be connected.
This meant:

1. We wouldn't fire a second event when transitioning to "really
connected". The only option for users was to listen on the old-style
per-connection notifications.
2. "Connectedness" checks would be a little too eager to treat a peer as
connected.

For 99% of users, "transient" peers should be treated as disconnected.
So while it's technically a breaking change to split-out "transient"
connectivity into a separate state, I expect it's more likely to fix
bugs than anything.

Unfortunately, this change _did_ require several changes to go-libp2p
itself because go-libp2p _does_ care about transient connections:

1. We want to keep peerstore information for transient peers.
2. We may sometimes want to treat peers as "connected" in the host.
3. Identify still needs to run over transient connections.

fixes #2692