Design user API

[DifferentSC]

We need a good user API for the system. Particularly, we need to hide the internal implementation of MIST including REEF components. I think we should also hide the data flow model below streaming queries and provide data-centric interface for users.

To be more specific, we need to provide those three features below via our MIST API.

• Data fetching from diverse sources (e.g. HDFS, KAFKA, ...)
• Data transformation within queries. It should be explicit and general enough to support various operations.
• Convenient output of query results

Issue #3 should be addressed before this issue.

[DifferentSC]

This issue has been splitted, so close this issue