Kill foreground event handlers on timeout

[hjoliver]

Close #6615

Shutdown handlers aren't run via the subprocess pool, which gets closed on shutdown, so they weren't subject to the process pool timeout. This PR imposes
the timeout on them anyway, outside of the pool.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Result, using the example from the issue: