fix(swarm): prevent overflow in keep-alive computation

[thomaseizinger]

Description

When adding a very large Duration to an Instant, an overflow can occur. To fix this, we check this before instantiating Delay and half the given duration until it no longer overflows.

Fixes: #4555.

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]

Feel free to release right away.

[thomaseizinger]

Just released! Thanks for the fix Max :)