add flush_at_shutdown option to buf_memory

This is just same thing with one which buf_file has https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/buf_file.rb#L180

I want this option because I do not need to flush all buffers at shutdown on my system, want to just discard them. 

Unlike buf_file, flush_at_shutdown is true as default for lower version compatibility.