[SPARK-22908] Add kafka source and sink for continuous processing.
## What changes were proposed in this pull request?
Add kafka source and sink for continuous processing. This involves two small
changes to the execution engine:
* Bring data reader close() into the normal data reader thread to
[SPARK-22908] Add kafka source and sink for continuous processing.
## What changes were proposed in this pull request?
Add kafka source and sink for continuous processing. This involves two small
changes to the execution engine:
* Bring data reader close() into the normal data reader thread to
