Make queue position refresh optional

[jyu00]

Summary

Introduce a new refresh parameter in job.queue_position() and query the API only if refresh=True.

Details and comments

#329 updated job.queue_position() to query the latest queue position every time it's called (by calling job.status()), with the intent to provide the caller the latest position. This change, however, severs the ties between job status and queue position. For example, job_monitor in qiskit-terra has:

if status.name == 'QUEUED':
    msg += ' (%s)' % job.queue_position()

but the job status may no longer be QUEUED when queue_position() is called, and queue_position() would return None.

To keep backward compatibility for users who rely on status and queue_position being in sync, this PR refreshes queue position only if refresh=True.