[ray] new worker create model

[chaokunyang]

Is your feature request related to a problem? Please describe.
Currently mars on ray cluster initialization process is as follows:

• create RayMainPool actor for supervisor
• start oscar service for supervisor
• create workers
  • create ray RayMainPool actor for worker
  • ray RayMainPool actor create RaySubPool actors
• start oscar service for workers

This process has some issues:

• When a main pool failed and restarted by ray, it will create new RaySubPool actors rather than restart previous RaySubPool actors, which makes actor management tricky and not easy to track failed process.
• Currently ray create main pool first, then main pool create subpools. Because import mars takes 2~3s, create a worker will take double times.

Describe the solution you'd like
It would be better if we can create all ray main/sub pool actors simultaneously, then initialize all mars services in all ray actors. In this way, mars cluster initialization time will be reduced to half of previous.
At the same time, if a ray main pool failed, all started mainpool and subpools will be the same actors as before.