[runner] Job execution pull or push

[at15]

Pull

• every time node heartbeat it tells job status on that node to central
• in response of heartbeat, central tell the node new job of next stage of current job

Push

• after a job stage is finished, node call central to tell this update
• after central collect enough information of one stage, it tells all node to proceed to next stage

[at15]

might just use push for now, it's easier ....