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Member fields of ExecutionVertex need bigger reconstruction.So change the currentExecution in ExecutionVertex to an ArrayList named executionList(The purpose of using list is to increase the code extensibility, in case that multiple speculative executions will run at the same time in the future), which means that there can be multiple executions in an ExecutionVertex at the same time.For each execution in executionList there is no difference of the behavior such as failover, request slot etc between them.

另一种方法，不改list，而是加一个speculativeExecution，说下不好的地方。

private List<Execution> executionList = new ArrayList<>();

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