rebuild worker list set strings pid not numbers

[killmenot]

@robtweed

Please take a look at https://github.com/robtweed/ewd-qoper8/blob/master/lib/master/proto/startWorker.js#L127-L130

I think

  this.worker.list = [];
  for (var pid in this.worker.process) {
    this.worker.list.push(parseInt(pid, 10));
  }

is more correct.

Affected methods: stopWorker, startWorker