Terminate worker tasks on SIGINT

[nkaretnikov]

Fixes #707.

Description

This pull request: installs a signal handler that terminates worker tasks on Ctrl-C (SIGINT).

If the requested signal isn't supported on the current platform, the operation will be ignored.

Notes:

• we cannot use celery control because it's broker-dependent, which wouldn't work with standalone
• printing a worker pid and asking users to terminate that wouldn't work because celery beat would respawn
• using install_worker_term_hard_handler wouldn't work because not all tasks would be terminated
• using a signal handler is the only solution I've found given the above constraints, it's what install_worker_term_hard_handler uses internally as well.

Tested on Windows and Linux. All processes are terminated as expected.

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

Additional information

[netlify]

✅ Deploy Preview for kaleidoscopic-dango-0cf31d canceled.

Name	Link
🔨 Latest commit	0237f94
🔍 Latest deploy log	https://app.netlify.com/sites/kaleidoscopic-dango-0cf31d/deploys/6596fe8bd78e4a0008c494a4

[trallard]

@nkaretnikov there is a merge conflict - can you fix this and we can merge right away