Promise Worker Pool

Implement createPool(worker, concurrency) that creates a pool of reusable workers with limited concurrency.

Requirements

• worker(task) is an async function that processes a task.
• submit(task) — submit a task to the pool. Returns a promise that resolves with the worker's result. If all workers are busy, the task waits in a queue.
• drain() — returns a promise that resolves when all submitted tasks have completed.
• At most concurrency tasks run simultaneously.

Example

const pool = createPool(async (n) => n * 2, 2);
const r1 = pool.submit(5);  // starts immediately
const r2 = pool.submit(10); // starts immediately
const r3 = pool.submit(15); // queued, waits for a slot
await pool.drain();