[DISCUSS] using kafka asynchronous producer is better than timer.at for kafka-logger plugin?

[~Jarvis.Qiu]

the code in: 
https://github.com/apache/incubator-apisix/blob/master/lua/apisix/plugins/kafka-logger.lua#L101

The limit of lua_max_pending_timers and lua_max_running_timers that may be 
reached.