swss: flush g_asicState after each event is done (sonic-net#570)

* swss: flush g_asicState after each event is done
* add flush() after event is handled in case some entries are still in buffer, don't wait
* with the changes in sairedis and swss-common, route performance improved by 200~300 routes/sec
* swss-common: remove unnecessary flush() in timeout case and update comment
* remove unnecessary flush() in timeout case and update comment

orchagent/orchdaemon.cpp

@@ -300,12 +300,6 @@ void OrchDaemon::start()
 
         if (ret == Select::TIMEOUT)
         {
-            /* Let sairedis to flush all SAI function call to ASIC DB.
-             * Normally the redis pipeline will flush when enough request
-             * accumulated. Still it is possible that small amount of
-             * requests live in it. When the daemon has nothing to do, it
-             * is a good chance to flush the pipeline  */
-            flush();
             continue;
         }
 
@@ -319,5 +313,12 @@ void OrchDaemon::start()
         for (Orch *o : m_orchList)
             o->doTask();
 
+        /* Let sairedis to flush all SAI function call to ASIC DB.
+         * Normally the redis pipeline will flush when enough request
+         * accumulated. Still it is possible that small amount of
+         * requests live in it. When the daemon has finished events/tasks, it
+         * is a good chance to flush the pipeline before next select happened.
+         */
+        flush();
     }
 }