Fix -- in extreme circumstances, messages can leave behind in the send buffer until the next message sending.
Fix -- basic_buffer doesn't support fixed arrays (just in the constructor) in GCC and Clang.
Introduce macro ASCS_CAN_EMPTY_NOT_SAFE for containers used in send/recv buffer whose empty function is not thread safe.

Add downgrade (to normal UDP) test for reliable UDP in udp_test.

For recv_msg, go back to dispatch_strand if macro ASCS_PASSIVE_RECV is not defined because it
 cannot be invoked concurrently without the macro.

More guarantee on object reusing.
A small refactoring.

Fix -- in Windows, a TCP client must explicitly specify a full IP address (not only the port) to connect to.

Fix bug in file_server/file_client -- put file can corrupt the target file if it already exist before the transmission.

Fix bug -- if binding local address failed, which means start socket failed, but socket::started() will still return true.

Enhance basic_buffer -- make it to be able to work as std::string during handling the end '\0' character although with different way.

Rollback the support of strict reference balance on multiple io_context, see the release notes for more details,
 so virtual function change_io_context been dropped.
server_base(acceptor) supports the reference customization of io_context.
timer supports the reference customization of io_context, almost all classes inherit from it.

Fix bugs:
 core dump happen in stop_service on restarted service_pump.
 macro ASCS_AVOID_AUTO_STOP_SERVICE will not take effect after service_pump been restarted.
 with macro ASCS_CLEAR_OBJECT_INTERVAL, after stop_service, some sockets can still exist in the valid object queue.
 server_base refuses to start listening if user has opened the acceptor before (to configure the acceptor).
 ssl::single_client_base cannot re-connect to the server.
 server_base's acceptor wrongly takes 2 references on 2 io_contexts.
 Drop function service_pump::get_executor, it introduces above bug implicitly.
 echo_server, echo_client and file_client cannot work after restarted the service_pump.

Keep the capacity unchanged in basic_buffer::clear as std::string::clear does.
Make basic_buffer copyable.