Harmonize concepts and identifiers with net-ts (API Change):

fix #321

This synchronizes identifier names and some implementation details
with the Networking-TS document and reference implementation.
The following interfaces have changed:

* async_completion
* is_buffer_sequence (removed)
* is_const_buffer_sequence (was is_ConstBufferSequence)
* is_dynamic_buffer (was is_DynamicBuffer)
* is_mutable_buffer_sequence (was is_MutableBufferSequence)
* async_result (new)
* BEAST_HANDLER_TYPE macro (new)
* BEAST_INITFN_RESULT_TYPE macro (new)

Filename changes:

* async_result.hpp (was async_completion.hpp)

test/core/async_result.cpp

@@ -0,0 +1,9 @@
+//
+// Copyright (c) 2013-2017 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/async_result.hpp>