fix#773
* buffer_cat is renamed to buffers_cat
* buffer_cat_view is renamed to buffers_cat_view
Actions Required:
* Use buffers_cat instead of buffer_cat
* Use buffers_cat_view instead of buffer_cat_view
* HTTP read and write operations now return the
number of bytes transferred to or from the stream.
Actions Required:
* Modify HTTP read and/or write completion handlers to receive
the extra std::size_t bytes_transferred parameter.
