title: Self referencing fields in protobuf messages
body: <p>Is it ok to have message like this?</p>
  
  <pre><code>message A {
    required int64 some_number = 1;
    // .... some more fields
    optional A sub_a = 123;
  }
  </code></pre>
  
  <p>The reason is my current protocol stores set of A directly, and wrapping A's in another message will lead to massive conversions of stored data.</p>
  
  <p>2.2.0 protoc compiles it ok. 
  Can this make any problems with serialization/deserialization, and is it supported by protobuf-net. </p>