use unpack syntax

Rather than defining a variable and then reading it from a message,
we also supports directly unpack-ing from the message.  Use this
syntax instead as it is more efficient and succinct.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2ce2aa3e6a43d8866ba9b3920dd3165b7b6b9f6e
diff --git a/example/asio-example.cpp b/example/asio-example.cpp
index 22ae70b..bc6249b 100644
--- a/example/asio-example.cpp
+++ b/example/asio-example.cpp
@@ -315,8 +315,8 @@
             std::cerr << "error with async_send\n";
             return;
         }
-        GetSubTreeType data;
-        ret.read(data);
+        auto data = ret.unpack<GetSubTreeType>();
+
         for (auto& item : data)
         {
             std::cout << item.first << "\n";