Make NginxMessageHandler inherit to a new class SystemMessageHandler. Move SharedCircularBuffer related operations there.
Merge remote-tracking branch 'origin/trunk-tracking' into xqyin-refactor-message-handler2
Make NgxMessageHandler inherit to SystemMessageHandler. Move SharedCircularBuffer things to SystemMessageHandler.
Merge pull request #733 from pagespeed/xqyin-refactor-message-handler3

Make NgxMessageHandler inherit to SystemMessageHandler