Amend basic_buffer's reserve concept (to be the same as std::string).
Enhance the default packer and unpacker to support basic_buffer as their output message type.
Enhance packer2 and unpacker2 to support unique_buffer<basic_buffer> and shared_buffer<basic_buffer> as their output message type.
To use the original default packer and unpacker, now you must write them as packer<> and unpacker<>.

Simplify class basic_buffer (doesn't support capacity concept anymore).
flexible_unpacker support basic_buffer as its message type.

Support UNIX domain socket.
Use std::list instead of ascs::list except the container type of the input queue and out queue in ascs::socket.

Rename auto_buffer to unique_buffer.

Optimize unpacker2.

Add a new parameter prior to send_msg (series) function (after can_overflow), if set to true, message will be inserted into the front of the send buffer.
Withdraw commit 976c3817.

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.

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.

direct_send_msg support batch operation.
Introduce virtual function type_name() and type_id() to ascs::socket, they can identify whether a given two ascs::socket has the same type.
Drop the native parameter from i_packer::pack_msg(msg_type&& msg, ...).

Delegate supression of memory replication to packers, so rename i_packer::pack_header to another overload of pack_msg.
See built-in packers for more details.

Make client_socket_base be able to call multi_client_base (via i_matrix) like server_socket_base call server_base (via i_server),
 and so does ssl::client_socket_base and udp::socket_base.

Fix statistics for batch message dispatching.
Hide all member variables for developers.

Demonstrate how to consume part of messages in on_msg_handle (echo_server).

Generalize function send_msg, send_native_msg, safe_send_msg, safe_send_native_msg, broadcast_msg, broadcast_native_msg, safe_broadcast_msg and safe_broadcast_native_msg.