Previously, a `ConnectionHandler` was immediately requested from the `NetworkBehaviour` as soon as a new dial was initiated or a new incoming connection accepted.
With this patch, we delay the creation of the handler until the connection is actually established and fully upgraded, i.e authenticated and multiplexed.
As a consequence, `NetworkBehaviour::new_handler` is now deprecated in favor of a new set of callbacks:
- `NetworkBehaviour::handle_pending_inbound_connection`
- `NetworkBehaviour::handle_pending_outbound_connection`
- `NetworkBehaviour::handle_established_inbound_connection`
- `NetworkBehaviour::handle_established_outbound_connection`
All callbacks are fallible, allowing the `NetworkBehaviour` to abort the connection either immediately or after it is fully established. All callbacks also receive a `ConnectionId` parameter which uniquely identifies the connection. For example, in case a `NetworkBehaviour` issues a dial via `NetworkBehaviourAction::Dial`, it can unambiguously detect this dial in these lifecycle callbacks via the `ConnectionId`.
Finally, `NetworkBehaviour::handle_pending_outbound_connection` also replaces `NetworkBehaviour::addresses_of_peer` by allowing the behaviour to return more addresses to be used for the dial.
Resolves#2824.
Pull-Request: #3254.
We create the `ConnectionId` for the new connection as part of `DialOpts`. This allows `NetworkBehaviour`s to accurately track state regarding their own dial attempts.
This patch is the main enabler of https://github.com/libp2p/rust-libp2p/pull/3254. Removing the `handler` field will allow us to deprecate the `NetworkBehaviour::new_handler` function in favor of four new ones that give more control over the connection lifecycle.
Previously, we used the full reference to the `OutEvent` of the `ConnectionHandler` in all implementations of `NetworkBehaviour`. Not only is this very verbose, it is also more brittle to changes. With the current implementation plan for #2824, we will be removing the `IntoConnectionHandler` abstraction. Using a type-alias to refer to the `OutEvent` makes the migration much easier.
