Expose off-heap memory usage in various places

With SPARK-13992, Spark supports persisting data into off-heap memory, but the usage of off-heap is not exposed currently, it is not so convenient for user to monitor and profile, so here propose to expose off-heap memory as well as on-heap memory usage in various places:

1. Spark UI's executor page will display both on-heap and off-heap memory usage.
2. REST request returns both on-heap and off-heap memory.
3. Also these two memory usage can be obtained programmatically from SparkListener.