Notifications System core

Following https://issues.cask.co/browse/CDAP-849 jira and design doc.
Build is running at https://builds.cask.co/browse/CDAP-DUT320-3.
No they are not. Metrics in local mode doesn't need Kafka (see `MetricsProcessorModule` and `KafkaMetricsProcessorModule`). It's similar for the log saver case.