Initial happy-path restart test (black-box)

This is a happy-path black box test. It starts a 2-worker cluster, then requests a restart. It asserts that a long-running request initiated before the restart completes successfully after the new workers are available, and that a request issued after the restart is handled by the new workers.

WIP: making sure that the old workers exit after closing out their active requests.

To achieve this, we use a custom worker script that, once listening, 'phones home' to a very small Express app that is set up by the test, so the workers can report on their status and be instructed to perform certain actions. With this in place we can drive out many different scenarios of worker behavior.

The tests are built using [js-csp](https://github.com/ubolonton/js-csp) and ES6 (compiled during testing) for the most expressive way to represent the complex asynchronous interactions.

@mdimas @quaelin