[SPARK-4062][Streaming]Add ReliableKafkaReceiver in Spark Streaming Kafka connector

Add ReliableKafkaReceiver in Kafka connector to prevent data loss if WAL in Spark Streaming is enabled. Details and design doc can be seen in [SPARK-4062](https://issues.apache.org/jira/browse/SPARK-4062).
What is the point of calling onStore data right after inserting into the buffer?