See T84723#1442170 for context.

There is a SIGINT handler in mw-ocg-service.js, but we need to add a SIGTERM handler. The SIGTERM handler should send a message to the child to ask it to exit cleanly at the end of its next loop. The backend server in particular needs a way to interrupt its redis-queue-fetch loop, but it would also be nice to check that the frontends aren't in the middle of handling a web request before killing them.