Add config for concurrent_task in executor

[Ted-Jiang]

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

        // Use a dedicated executor for CPU bound tasks so that the main tokio
            // executor can still answer requests even when under load
            // TODO make it configurable
            let dedicated_executor = DedicatedExecutor::new("task_runner", 4);

Before in DedicatedExecutor pool is fixed size 4 thread,
When we facing large task set, it will cut down the system throughput.
Need align with scheduler_config_spec.toml

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