[Ballista] Load testing for Push-based task scheduling get stuck

[yahoNanJing]

Describe the bug

There are two bugs.

• Losing the event SchedulerServerEvent::JobSubmitted results in job no longer to be scheduled
• Concurrency issue of updating ExecutorData simultaneously.

1. For the first bug:
   In the method of SchedulerServerEventAction::offer_resources, the returned available_executors may be all with 0 available_task_slots. In this case, there'll be no tasks to be scheduled for the job and no SchedulerServerEvent::JobSubmitted will be resent to the channel. As a result, the job will get stuck.

2. For the second bug:
   The operations of get_executor_data and save_executor_data are not atomic, which may result in concurrency issue.

To Reproduce

Run loading test with Push-based task scheduling policy as described in #1983.

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
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