title: best way to pass results of one forkJoin to another via http
body: <p>I need to make multiple http calls (angular) and return the results of one forkJoin to another.
  the following will not work:</p>
  
  <pre><code>Observable.forkJoin([
         this.userService.createAccountHttp(accountRequest),
         this.userService.verifyAccountHttp(res)
   ]);
  
  createAccount() {
    return this._http.jsonp&lt;any[]&gt;('http://...', 'callback').pipe(
        map(v =&gt; {
          return v;
        })
  }
  verifyAccountHttp(res) {
    return this._http.jsonp&lt;any[]&gt;('http://some url/' + res, 'callback').pipe(
        map(v =&gt; {
          return v;
        })
  }
  
  </code></pre>
  
  <p>as res is undefined.
  Is there a way to use forkJoin to run sequential http calls and pass results of one http call to the next?</p>
  
  <p>Thanks</p>