MetricsSystem refactor

core/src/main/scala/spark/deploy/master/Master.scala

...ain/scala/spark/deploy/master/MasterInstrumentation.scala

core/src/main/scala/spark/deploy/worker/Worker.scala

...ain/scala/spark/deploy/worker/WorkerInstrumentation.scala

core/src/main/scala/spark/metrics/MetricsSystem.scala

core/src/main/scala/spark/metrics/sink/ConsoleSink.scala

core/src/main/scala/spark/metrics/sink/CsvSink.scala

core/src/main/scala/spark/metrics/sink/JmxSink.scala

core/src/main/scala/spark/metrics/sink/Sink.scala

core/src/main/scala/spark/metrics/source/JvmSource.scala

core/src/main/scala/spark/metrics/source/Source.scala