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.

Introduce virtual function change_io_context() to ascs::socket to balance the reference of io_context strictly.

Suppress error logs for empty heartbeat (suppose you want to stop heartbeat but keep heartbeat checking).

Add new macro ASCS_ARBITRARY_SEND.