title: TPL DataFlow, link blocks with priority?
body: <p>Using TPL.DataFlow blocks, is it possible to link two or more sources to a single ITargetBlock(e.g. ActionBlock) and prioritize the sources?</p>
  
  <p>e.g.</p>
  
  <pre><code>BufferBlock&lt;string&gt; b1 = new ...
  BufferBlock&lt;string&gt; b2 = new ...
  ActionBlock&lt;string&gt; a = new ...
  
  //somehow force messages in b1 to be processed before any message of b2, always
  b1.LinkTo (a);
  b2.LinkTo (a);
  </code></pre>
  
  <p>As long as there are messages in b1, I want those to be fed to "a" and once b1 is empty, b2 messages are beeing pushed into "a"</p>
  
  <p>Ideas?</p>