[Blimp Net] Add EngineAuthHandler.

blimp/net/connection_handler.h

Index: blimp/net/connection_handler.h
diff --git a/blimp/net/connection_handler.h b/blimp/net/connection_handler.h
index b9d37f6199163030cd2fff0de551bda0a49eab72..afce887942c8af0f12ad45db3b71ccf3d5bdfe05 100644
--- a/blimp/net/connection_handler.h
+++ b/blimp/net/connection_handler.h
@@ -14,6 +14,9 @@ class ConnectionHandler {
  public:
   virtual ~ConnectionHandler() {}
 
+  // The receiving object is responsible for synchronously attaching its
+  // necessary delegates and observers to |connection| during the call to
+  // HandleConnection.

[Wez, 2015/12/04 01:35:38]

I'm not keen on this - it means that all ConnectionHandlers have to know what
"necessary delegates and observers" are - if we added some new "necessary
delegate" then old ConnectionHandler implementations should break if this
comment holds true.

Why is it problematic to have a temporarily-unattached |connection|, or to wait
to attach handlers, etc, to a connection until later on?

[haibinlu, 2015/12/04 02:50:40]

I do not like this either. It is the internal implementation detail of
BlimpConnection that forces such requirement:  

Because BlimpConnection's incoming processor can not be set to nullptr, when a
BlimpConnection is hand over from one connection handler to another, the
connection may deliver message to an obsolete message processor; requiring a
synchronous updating incoming message processor prevents this from happening.

I prefer to restore the ability to set BlimpConnection's incoming processor to
nullptr, so that auth handler can use it to stop receiving further messages. 

Kevin has different opinion on this. 

   virtual void HandleConnection(scoped_ptr<BlimpConnection> connection) = 0;
 };