Adds getExecutor to Çlosure

[killme2008]

Currently, JRaft uses a single global executor for all closures, including user task closures.

We can add a getExecutor method to the Closure interface:

interface Closure {
    default ExecutorService getExecutor() {
        return null;
    }
}

If this method returns a non-null executor, use it instead of the global executor.

Additionally, we could split executors by task type to improve isolation and performance.