Use std::forward_list instead of std::list, and implement size concept like boost::container::slist does.

Introduce some helper classes (ascs::socket2, socket3 and socket4) to reduce the number of template parameters.
Refactoring: move unpacker and strand from tcp::socket_base and udp::socket_base to ascs::socket.

Fix bug: if virtual bool on_msg_handle(OutMsgType&) returns false (means the message is not handled), the message will lost and never be re-dispatched.
Member variables refactoring.

Demonstrate how to use single_service in demo echo_server.

Rename udp::multi_service_base to multi_socket_service_base.
Rename ext::udp::single_service to single_socket_service.
Rename ext::udp::multi_service to multi_socket_service.
Rename ext::udp::service to socket_service.

Drop the 'reset' parameter in multi_socket_service::add_socket, this means never call reset() for the socket anymore.
Don't call reset() in ascs::single_socket_service::init() and ascs::multi_socket_service::init() anymore.

Fix bug: queue::size_in_byte() may return wrong data with macro ASCS_SYNC_RECV or ASCS_SYNC_DISPATCH.

Fix infinite waiting after sync message sending or receiving failed.

Rename replaceable_unpacker to unpacker2, replaceable_udp_unpacker to udp_unpacker2, replaceable_packer to packer2,
 because their names confuse users, any packer or unpacker is replaceable for those packer or unpacker that has the same msg_type.