title: Add queueing to Angular's HttpClient
body: <p>I have exact same requirement as mentioned in <a href="https://stackoverflow.com/questions/14464945/add-queueing-to-angulars-http-service">Add queueing to angulars $http service</a> but need implementation in Angular 4.3 or 5 using the <code>HttpInterceptor</code> from <code>@angular/common/http</code>.</p>
  
  <p>I have a very quirky API that can only handle a single request at a time for a particular browser session. Therefore, I need to ensure that every time a request is made in same session, it goes into a queue, and that queue is executed one request at a time, until it is empty.</p>