title: Akka streams combine sources, sources appear to start simultaneously
body: <p>Im using  Source.combine in Akka streams.  I noticed that all of the sources appear to start at the same time at the beginning of the graph lifecycle and buffer some input before pausing and resuming in the order of the source.combine list.  </p>
  
  <pre><code>val singletonSources = builder.add(
    Source.combine( 
      Source.single(("TABLE HEADER")),             
      actorSource1.map(_ ... toString), //a source using ask ? to an actor                       
      Source.single("ETC"),             
      HttpSource1.map(x=&gt; bytes... to string),                     
      Source.single("ETC"),             
      actorSource2.map(_ ... toString), //a source using ask ? to an actor                  
      Source.single("ETC"),             
      HttpSource2.map(x=&gt; bytes... to string),              
    )(Concat(_))) 
  </code></pre>
  
  <p>I have sources coming from HttpRequests as well as actors with ask ? but both types appear to "buffer" some input when the graph is run.  I do see they each wait to resume in the correct order but for http requests, unless I put them at the beginning of the sources list before long running requests I get an Akka tcp timeout exception as the connection appears to wait for the incoming data. </p>