sairedis: add while loop to make syncd processEvent() handle as many …

…entries as possible in one event call (sonic-net#335)

* sairedis: add while loop to make syncd processEvent() handle as many entries as possible in one event call
* add while() loop to handle entries when buffer is Not empty
* this change has dependency on the added empty() function in ConsumerTableBase class in swss-common which is submitted in other request
* with the changes in swss-common, route performance improved about ~200 routes/sec
*   sairedis: update SWSS_LOG_INFO to SWSS_LOG_DEBUG
  * update SWSS_LOG_INFO to SWSS_LOG_DEBUG