Logging in Watchbot4

[tapaswenipathak]

Closes #204.

cc @mapbox/platform-engine-room.

[jakepruitt]

Per conversation, we will need to create two different loggers inside of the lib/worker.js:

constructor(message = {}, options = {}) {
  // ...

  this.workerLogger = Logger.create('worker', message);
  this.watcherLogger = Logger.create('watchbot', message);
}

The workerLogger will be used within the child method as the destination of the child.stdout.pipe(workerLogger.stream()), that way all child logs will be prefixed with worker.

The watcherLogger will be used everywhere else in the file, like the success and failure messages that are meta information surrounding the child process.