Fix the infinite loop created by setImmediate() in threads example

[m4heshd]

This is an update to the master.js used in worker threads example.

Issue:

The if condition that checks for pending jobs wrapped in the poll() function creates an infinite loop by calling itself using setImmediate(poll) when the job queue is empty. This creates a massive unnecessary workload for one or many cores in the CPU. (Comment in #237 )

function poll() {
    if (queue.length) {
        // If there's a job in the queue, send it to the worker
        job = queue.shift();
        worker.postMessage(job.message);
    } else {
        // Otherwise, check again later
        timer = setImmediate(poll); <- Gets looped infinitely if the queue has no jobs
    }
}

Fix:

Get rid of the setImmediate(poll) completely and introduce a function that schedules new jobs manually whenever asyncQuery() is called. Workers will remain idle rest of the time.

[JoshuaWise]

Thanks for improving this 👍

[m4heshd]

@JoshuaWise My pleasure and much thanks to you for creating this flawless tool. Using this feature with better-sqlite3 heavily right now. Hoping to contribute more.