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/http/nodejs_parser.hpp

@@ -482,7 +482,7 @@ std::size_t
 nodejs_basic_parser<Derived>::write(
     ConstBufferSequence const& buffers, error_code& ec)
 {
-    static_assert(beast::is_ConstBufferSequence<
+    static_assert(beast::is_const_buffer_sequence<
         ConstBufferSequence>::value,
             "ConstBufferSequence requirements not met");
     using boost::asio::buffer_cast;