Gracefully handle operator termination

[metacosm]

In particular, depending on how the operator exits, it could leave behind orphaned plugin processes. While this situation should be rare in normal operations, it's quite frequent during development so, ideally, we would gracefully handle terminations to make sure the plugin processes are properly terminated as well.

[metacosm]

Possible options:

• https://gobyexample.com/signals
• https://medium.com/@matryer/make-ctrl-c-cancel-the-context-context-bd006a8ad6ff
• https://golangcode.com/handle-ctrl-c-exit-in-terminal/