Add @dougal/concurrency module

lib/modules/@dougal/concurrency/index.js

@@ -0,0 +1,25 @@
+class ConcurrencyLimiter {
+
+  constructor(maxConcurrent) {
+    this.maxConcurrent = maxConcurrent;
+    this.active = 0;
+    this.queue = [];
+  }
+
+  async enqueue(task) {
+    if (this.active >= this.maxConcurrent) {
+      await new Promise(resolve => this.queue.push(resolve));
+    }
+    this.active++;
+    try {
+      return await task();
+    } finally {
+      this.active--;
+      if (this.queue.length > 0) {
+        this.queue.shift()();
+      }
+    }
+  }
+}
+
+module.exports = ConcurrencyLimiter;

lib/modules/@dougal/concurrency/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@dougal/concurrency",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "description": ""
+}