[SPARK-4857] [CORE] Adds Executor membership events to SparkListener

Adds onExecutorAdded and onExecutorRemoved events to the SparkListener. This will allow a client to get notified when an executor has been added/removed and provide additional information such as how many vcores it is consuming.

In addition, this commit adds a SparkListenerAdapter to the Java API that provides default implementations to the SparkListener. This is to get around the fact that default implementations for traits don't work in Java. Having Java clients extend SparkListenerAdapter moving forward will prevent breakage in java when we add new events to SparkListener.

Author: Kostas Sakellis <kostas@cloudera.com>

Closes #3711 from ksakellis/kostas-spark-4857 and squashes the following commits:

946d2c5 [Kostas Sakellis] Added executorAdded/Removed events to MesosSchedulerBackend
b1d054a [Kostas Sakellis] Remove executorInfo from ExecutorRemoved event
1727b38 [Kostas Sakellis] Renamed ExecutorDetails back to ExecutorInfo and other CR feedback
14fe78d [Kostas Sakellis] Added executor added/removed events to json protocol
93d087b [Kostas Sakellis] [SPARK-4857] [CORE] Adds Executor membership events to SparkListener