Move to modern asio interfaces

async_result is required to be able to support boost 1.70.  This commit
moves us to it.

Tested:
Compiled.  Changes are non-functional, and simply renamed in newer
versions of asio.

Change-Id: I46728dc146c4c49eff7907c6aa8bc1525eca5cc2
Signed-off-by: Ed Tanous <ed.tanous@intel.com>
diff --git a/sdbusplus/asio/connection.hpp b/sdbusplus/asio/connection.hpp
index bafa4c6..8590627 100644
--- a/sdbusplus/asio/connection.hpp
+++ b/sdbusplus/asio/connection.hpp
@@ -88,9 +88,9 @@
         boost::asio::async_completion<
             MessageHandler, void(boost::system::error_code, message::message)>
             init(handler);
-        detail::async_send_handler<typename boost::asio::handler_type<
-            MessageHandler,
-            void(boost::system::error_code, message::message)>::type>(
+        detail::async_send_handler<typename boost::asio::async_result<
+            MessageHandler, void(boost::system::error_code,
+                                 message::message)>::completion_handler_type>(
             std::move(init.completion_handler))(get(), m);
         return init.result.get();
     }