fix(relay/client): only try to forward handler events once

[elenaf9]

Description

We currently try to forward pending handler events in the relay client behavior twice: in handle_established_{in, out}bound_connection and in on_swarm_event() (introduced in #3328).
This is redundant because by the second time we try to forward it there won't be any pending event anymore.
This PR removes the duplicated logic from on_swarm_event. Forwarding it in handle_established_{in, out}bound_connection is more convenient as we still have direct access to the handler there and don't need to go through the swarm.

However, it requires that in handle_established_{in, out}bound_connection the event is also removed when the connection is relayed, in which case the event is simply discarded because the commands are only valid on direct connections.

Notes & open questions

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