[SPARK-19358][CORE] LiveListenerBus shall log the event name when dropping...

[SPARK-19358][CORE] LiveListenerBus shall log the event name when dropping them due to a fully filled queue

## What changes were proposed in this pull request?

Some dropped event will make the whole application behaves unexpectedly, e.g. some UI problem...we shall log the dropped event name to facilitate the debugging

## How was this patch tested?

Existing tests

Author: CodingCat <zhunansjtu@gmail.com>

Closes #16697 from CodingCat/SPARK-19358.

core/src/main/scala/org/apache/spark/scheduler/LiveListenerBus.scala

@@ -232,6 +232,7 @@ private[spark] class LiveListenerBus(conf: SparkConf) extends SparkListenerBus {
         "This likely means one of the SparkListeners is too slow and cannot keep up with " +
         "the rate at which tasks are being started by the scheduler.")
     }
+    logTrace(s"Dropping event $event")
   }
 }