exp queue: start/stop queue processing

[pmrowla]

Implement basic CLI support for starting and stopping processing of the exp queue.

• dvc queue start: start queue processing
  • Should support setting an initial -j/--jobs concurrency value
  • Subsequent calls should do nothing (other than informational messaging that queue is already running)
• dvc queue stop: stop queue processing
  • Should use celery shutdown to stop queue processing (after currently running experiments complete)
  • Likely requires setting up a celery signal handler that runs when our exp cleanup tasks complete, and broadcasting the worker shutdown from there (since by default celery exits after completing the current task but we want it to exit after completing the current task chain)