feat(relay): emit event when all client connections are dropped

[RolandSherwin]

Description

When a relay server has no more connection with a reserved client, it would remove the reservation and the drop the circuits without any information passed to the server. It will be useful to for a server to track all its reservations and to know when they're removed (without keeping track of the connections themselves).

This PR aims to notify the server when a reservation closes, with the emission of the following event

    /// A reservation has been closed.
    ReservationClosed { src_peer_id: PeerId },

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[elenaf9]

Suggested change

	peer.get_mut().remove(&connection_id);
	if peer.get().is_empty() {
	peer.remove();
	self.queued_actions
	.push_back(ToSwarm::GenerateEvent(Event::ReservationClosed {
	src_peer_id: peer_id,
	}));
	}
	if peer.get_mut().remove(&connection_id) {
	self.queued_actions
	.push_back(ToSwarm::GenerateEvent(Event::ReservationClosed {
	src_peer_id: peer_id,
	}));
	}
	if peer.get().is_empty() {
	peer.remove();
	}

We may have multiple connections to a peer, so we should emit the event when a connection closes for which we have an active reservation.

Maybe it also makes sense to include the connection id in the event?

[dariusc93]

I agree. Additionally, we should also include the connection id of which we had the reservation since there may be multiple reservation to a peer with different connections.

[b-zee]

To make it complete, the other events should also have connection IDs: ReservationReqAccepted, ReservationReqDenied and ReservationTimedOut.

[elenaf9]

How about we keep the ReservationClosed variant without the connection-id in this PR, and do a separate one that adds it for all of the mentioned variants (and discuss there if we want to in the first place)? That way it's consistent.