Always define `start` and `stop` on `Service` #48

`Service`'s types say that `start` and `stop` can be undefined. This is impossible in practice because the only way to instantiate a `Service` is via `Registry.prototype.publish`, which always sets these properties. This small refactor avoids this problem to ensure that `start` and `stop` are always defined, which should make the library a bit easier to use.