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

• [x] 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

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[trallard]

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