Replace SocketAdaptors with asio tcp and ssl sockets.

[kenba]

The HTTP connection class is only used by the tcp_adaptor and ssl_tcp_adaptor which are simply wrappers around the ASIO::ip::tcp::socket and ASIO::ssl::streamASIO::ip::tcp::socket respectively.

The overall design could be simplified by redesigning the connection class to use the asio tcp and ssl sockets directly.