instruction: Write Go code to go by example: worker pools
output: ```dockerfile
  $ time go run worker-pools.go
  worker 1 started job 1
  worker 2 started job 2
  worker 3 started job 3
  worker 1 finished job 1
  worker 1 started job 4
  worker 2 finished job 2
  worker 2 started job 5
  worker 3 finished job 3
  worker 1 finished job 4
  worker 2 finished job 5
  ```