title: No FindAsync() method on IDbSet<T>
body: <p>Is there a reason that the <code>FindAsync()</code> method is omitted from the <code>IDbSet&lt;T&gt;</code> interface? <code>Find</code> is part of the interface, it seems odd the async version isn't available. I'm needing to cast to <code>DbSet&lt;T&gt;</code> to access it, which is a bit cumbersome:</p>
  
  <pre><code>User user = await ((DbSet&lt;User&gt;)db.Users)
      .FindAsync("de7d5d4a-9d0f-48ff-9478-d240cd5eb035");
  </code></pre>