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/buffer_test.hpp

@@ -18,7 +18,7 @@ namespace test {
 
 template<class ConstBufferSequence>
 typename std::enable_if<
-    is_ConstBufferSequence<ConstBufferSequence>::value,
+    is_const_buffer_sequence<ConstBufferSequence>::value,
         std::size_t>::type
 buffer_count(ConstBufferSequence const& buffers)
 {
@@ -27,7 +27,7 @@ buffer_count(ConstBufferSequence const& buffers)
 
 template<class ConstBufferSequence>
 typename std::enable_if<
-    is_ConstBufferSequence<ConstBufferSequence>::value,
+    is_const_buffer_sequence<ConstBufferSequence>::value,
         std::size_t>::type
 size_pre(ConstBufferSequence const& buffers)
 {
@@ -46,7 +46,7 @@ size_pre(ConstBufferSequence const& buffers)
 
 template<class ConstBufferSequence>
 typename std::enable_if<
-    is_ConstBufferSequence<ConstBufferSequence>::value,
+    is_const_buffer_sequence<ConstBufferSequence>::value,
         std::size_t>::type
 size_post(ConstBufferSequence const& buffers)
 {
@@ -58,7 +58,7 @@ size_post(ConstBufferSequence const& buffers)
 
 template<class ConstBufferSequence>
 typename std::enable_if<
-    is_ConstBufferSequence<ConstBufferSequence>::value,
+    is_const_buffer_sequence<ConstBufferSequence>::value,
         std::size_t>::type
 size_rev_pre(ConstBufferSequence const& buffers)
 {
@@ -70,7 +70,7 @@ size_rev_pre(ConstBufferSequence const& buffers)
 
 template<class ConstBufferSequence>
 typename std::enable_if<
-    is_ConstBufferSequence<ConstBufferSequence>::value,
+    is_const_buffer_sequence<ConstBufferSequence>::value,
         std::size_t>::type
 size_rev_post(ConstBufferSequence const& buffers)
 {