Implement asio dealloc-before-invoke guarantee:

fix #215

This change guarantees that temporary memory allocated
through the asio hooks by the Beast implementation is
deallocated before invoking the final handler when performing
composed operations.

The change is accomplished by replacing std::shared_ptr with
a thread-safe custom container handler_ptr to manage composed
operation state. The container tracks other instances which
manage the same object and resets them in a safe way before
invoking the final handler.

handler_ptr is provided as a public interface so that users of
this library can utilize the same idiom to write their own
composed operations.

test/core/handler_ptr.cpp

@@ -0,0 +1,9 @@
+//
+// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+// Test that header file is self-contained.
+#include <beast/core/handler_ptr.hpp>