[Ballista] Introduce StageManager for managing tasks stage by stage

[yahoNanJing]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

It's better to manage tasks by stages rather than mess them up for the whole system or the whole job. With Stage, it will be much easier to maintain the task status changes within a stage, which will also be easier for future task error handling. And it will also be easier to manage the stage status changing and error handling.

Describe the solution you'd like

• Introduce Stage for a bunch of tasks for a stage.
• Introduce StageManager for managing all of the stages.

With this change, it will be much efficient to fetch a task to be scheduled from some Stage. And it also makes it possible to schedule tasks by the priority of stages.

Additional context

#1704

[yahoNanJing]

There are three levels of job state: task -> stage -> job.

For the state machine of task:

For the state machine of stage:

For the state machine of job:

[yahoNanJing]

Sequential diagram for Pull-based task scheduling:

Sequential diagram for Push-based task scheduling: