fix#642
This changes websocket::stream close and async_close to automatically
read and discard incoming message frames before shutting down the
connection. It is no longer necessary to manually drain the connection.
fix#642
* Calls to stream::close and stream::async_close will
automatically perform the required read operations
Actions Required:
* Remove calling code which drains the connection after
calling stream::close or stream::async_close
* read and async_read now return the number of bytes inserted
into the caller's buffer.
Actions Required:
* Change the signature of completion handlers used with
websocket::stream::async_read to void(error_code, std::size_t)
