Hold a promise until signal caught, ensuring service remains running

[jaydeebee]

What did you implement:

Closes #104

How did you implement it:

Adds a new promise to the promise chains that handle long-running services. That promise fulfills when an exit signal (SIGTERM SIGINT) is received. This code is strongly inspired by similar code in serverless-offline.

How can we verify it:

Not sure. I couldn't figure out why the problem behavior (#104) started happening; given the fix, I'm surprised it doesn't happen more, and I expect there's something I'm missing. I'd love to talk to someone with more expertise, maybe we can figure that out.

Todos:

• [ ] Write tests
• [ ] Write documentation
• [ ] Fix linting errors
• [ ] Provide verification config/commands/resources
• [X] Change ready for review message below

Is this ready for review?: Sure?