Add fetch concurrency limit on obs annotation loading (#1318)

* add fetch concurrency limit

* add tests for PromiseLimit

client/src/util/promiseLimit.js

@@ -0,0 +1,50 @@
+/*
+This class provides a means to resolve Promises asynchronously,
+and limit the number concurrently executed.  For example, if
+you want to involve a large number of API endpoints using fetch(),
+but want to limit the number simultaneously outstanding.
+
+Example usage:
+
+const plimit = new PromiseLimit(2);
+return Promise.all([
+	plimit.add(() => fetch('/foo')),
+	plimit.add(() => fetch('/bar')),
+	plimit.add(() => fetch('/baz'))
+])
+*/
+export class PromiseLimit {
+	constructor(maxConcurrency) {
+		this.queue = new Set();
+		this.maxConcurrency = maxConcurrency;
+		this.pending = 0;
+	}
+
+	add(fn, ...args) {
+		// fn - must return a promise
+		// args - will be passed to fn
+		return new Promise((resolve, reject) => {
+			this.queue.add({ fn, args, resolve, reject });
+			this._resolveNext(false);
+		});
+	}
+
+	_resolveNext = (completed = true) => {
+		if (completed) this.pending -= 1;
+
+		while (this.queue.size > 0 && this.pending < this.maxConcurrency) {
+			const task = this.queue.values().next().value; // order of insertion
+			this.pending += 1;
+			this.queue.delete(task);
+			const { resolve, reject, fn, args } = task;
+
+			try {
+				const result = fn(...args);
+				result.then(this._resolveNext, this._resolveNext);
+				result.then(resolve, reject);
+			} catch (err) {
+				reject(err);
+			}
+		}
+	};
+}