title: RXJava execute more observables sequentially
body: <p><strong>I'm using Firebase to get objects a real time, I would like to execute  more observables sequentially (first in first out), thanks
  you!</strong></p>
  <pre class="lang-java prettyprint-override"><code>public void getDispatch(Dispatch dispatch) {
      Single&lt;DispatchAction&gt; observable =  Single.just(dispatch)
      .map(order1 -&gt; {
          return convertDispatch(order1);
      });
  
     DisposableSingleObserver&lt;DispatchAction&gt; actionDisposableObserver =
               new DisposableSingleObserver&lt;DispatchAction&gt;(){
          @Override
          public void onSuccess(DispatchAction action) {//Process result }
  
          public void onError(Throwable e) {//Error }
     };
  
     compositeDisposable.add(
          observable
          .subscribeOn(Schedulers.single())
          .observeOn(AndroidSchedulers.mainThread())
          .subscribeWith(actionDisposableObserver)
     );
  }
  </code></pre>