cat-log: add timeout to processes

[oliver-sanders]

• Partially addresses cat-log: clean up processes on websocket disconnect #643
• Add a timeout to cylc cat-log processes:
  • Ensures we don't accumulate them by accident.
  • Improves efficiency by closing long-lived log streams (the user probably isn't looking at the log file any more and the log file has probably stopped growing anyway).
• At present, cylc cat-log processes may be left behind if the websocket is closed abruptly. This has been observed as the result of a proxy timeout.

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]

(gah, the trouble orphaned cat-log processes have caused over the years!)

[wxtim]

Reasonable. Thank you for all the extra annotation. Some of this code is NOT self documenting IMO.

One Spelling error

[hjoliver]

Yep, good.