feat(swarm): don't have ConnectionHandlers close connections

[thomaseizinger]

Description

This PR implements the long-awaited design of disallowing ConnectionHandlers to close entire connections. Instead, users should close connections via ToSwarm::CloseConnection from a NetworkBehaviour or - even better - from the Swarm via close_connection. A NetworkBehaviour also does not have a "full" view onto how a connection is used but at least it can correlate whether it created the connection via the ConnectionId. In general, the more modular and friendly approach is to stop "using" a connection if a particular protocol no longer needs it. As a result of the keep-alive algorithm, such a connection is then closed automatically.

Depends-on: #4745.
Depends-on: #4718.
Depends-on: #4749.
Related: #3353.
Related: #4714.
Resolves: #3591.

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

[mxinden]

Let me know once this is ready for a review. Direction looking good.

[thomaseizinger]

Let me know once this is ready for a review. Direction looking good.

Needs all linked PRs to merge first before the build is green, otherwise ready for review :)

[thomaseizinger]

@mxinden All dependencies of this PR have been merged so this is ready for review.

[mergify]

This pull request has merge conflicts. Could you please resolve them @thomaseizinger? 🙏

[mxinden]

Great to see this final step.