shutdown: terminate subprocesses

[oliver-sanders]

• We use a subprocess pool for running backgorund commands (e.g. cylc clean).
• This creates subprocesses to run our commands in, however, it does not kill the subprocess once the command has completed, it keeps the subprocess open for future reuse (more efficient than creating and destroying them every time).
• On shutdown we need to close the pool to mop up these subprocesses (this doesn't happen automatically).

From experimentation, it would appear that these subprocess do die naturally when run in single-user mode (cylc gui), however, become orphan processes when run in muilti-user mode (cylc hub, tested with sudospawner).

Unfortunately, this does not clean up the dangling subprocesses in multi-user mode due to #392

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.

[oliver-sanders]

CI issues fixed in #620

[MetRonnie]

Kicking tests

[MetRonnie]

LGTM, haven't tested

[wxtim]

Tried it out, seems OK