Add queue libraries.

A basic queue implementation and one that consumes its items
automatically until empty.

lib/www/server/lib/queue/actions-queue.js

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+const Queue = require('./queue');
+
+// Inspired by:
+// https://stackoverflow.com/questions/53540348/js-async-await-tasks-queue#53540586
+
+class ActionsQueue extends Queue {
+
+	constructor (items = []) {
+		super(items);
+
+		this.pending = false;
+	}
+
+	enqueue (action) {
+		return new Promise ((resolve, reject) => {
+			super.enqueue({ action, resolve, reject });
+			this.dequeue();
+		});
+	}
+
+	async dequeue () {
+
+		if (this.pending) {
+			return false;
+		}
+
+		const item = super.dequeue();
+
+		if (!item) {
+			return false;
+		}
+
+		try {
+
+			this.pending = true;
+
+			const result = await item.action(this);
+
+			this.pending = false;
+			item.resolve(result);
+		} catch (err) {
+			this.pending = false;
+			item.reject(err);
+		} finally {
+			this.dequeue();
+		}
+
+	}
+
+}
+
+module.exports = ActionsQueue;

lib/www/server/lib/queue/index.js

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+
+module.exports = {
+	Queue: require('./queue'),
+	ActionsQueue: require('./actions-queue')
+};
+

lib/www/server/lib/queue/queue.js

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+
+class Queue {
+
+	constructor (items = []) {
+		this.items = items;
+	}
+
+	enqueue (item) {
+		this.items.push(item);
+	}
+
+	dequeue () {
+		return this.items.shift();
+	}
+
+	length () {
+		return this.items.length;
+	}
+
+}
+
+module.exports = Queue;