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Asynchronously shut down SSL on the stream.
template< typename ShutdownToken = default_completion_token_t<executor_type>> DEDUCED async_shutdown( ShutdownToken && token = default_completion_token_t< executor_type >());
This function is used to asynchronously shut down SSL on the stream. It is an initiating function for an asynchronous operation, and always returns immediately.
                The completion
                token that will be used to produce a completion handler, which
                will be called when the shutdown completes. Potential completion
                tokens include use_future, use_awaitable, yield_context, or a function
                object with the correct completion signature. The function signature
                of the completion handler must be:
              
void handler( const asio::error_code& error // Result of operation. );
                Regardless of whether the asynchronous operation completes immediately
                or not, the completion handler will not be invoked from within this
                function. On immediate completion, invocation of the handler will
                be performed in a manner equivalent to using post.
              
void(asio::error_code)
          This asynchronous operation supports cancellation for the following cancellation_type
          values:
        
cancellation_type::terminal
            cancellation_type::partial
            
          if they are also supported by the Stream
          type's async_read_some
          and async_write_some operations.