Throttle calls to HIBP api (#25)

Randomly failing to check by passwords, I'm pretty sure its because ~2000 connections are made at the same time.

src/misc/throttle.ts

@@ -0,0 +1,57 @@
+/**
+ * Use as a Decorator on async functions, it will limit how many times the function can be
+ * in-flight at a time.
+ *
+ * Calls beyond the limit will be queued, and run when one of the active calls finishes
+ */
+export function throttle(limit: number, throttleKey: (args: any[]) => string) {
+    return <T>(target: any, propertyKey: string | symbol,
+               descriptor: TypedPropertyDescriptor<(...args: any[]) => Promise<T>>) => {
+        const originalMethod: () => Promise<T> = descriptor.value;
+        const allThrottles = new Map<any, Map<string, Array<() => void>>>();
+
+        const getThrottles = (obj: any) => {
+            let throttles = allThrottles.get(obj);
+            if (throttles != null) {
+                return throttles;
+            }
+            throttles = new Map<string, Array<() => void>>();
+            allThrottles.set(obj, throttles);
+            return throttles;
+        };
+
+        return {
+            value: function(...args: any[]) {
+                const throttles = getThrottles(this);
+                const argsThrottleKey = throttleKey(args);
+                let queue = throttles.get(argsThrottleKey);
+                if (!queue) {
+                    queue = [];
+                    throttles.set(argsThrottleKey, queue);
+                }
+
+                return new Promise<T>((resolve, reject) => {
+                    const exec = () => {
+                        originalMethod.apply(this, args)
+                            .finally(() => {
+                                queue.splice(queue.indexOf(exec), 1);
+                                if (queue.length >= limit) {
+                                    queue[limit - 1]();
+                                } else if (queue.length === 0) {
+                                    throttles.delete(argsThrottleKey);
+                                    if (throttles.size === 0) {
+                                        allThrottles.delete(this);
+                                    }
+                                }
+                            })
+                            .then(resolve, reject);
+                    };
+                    queue.push(exec);
+                    if (queue.length <= limit) {
+                        exec();
+                    }
+                });
+            },
+        };
+    };
+}