Support reading/writing tiny RDBMS tables

It'd be quite helpful if Impala can read/write some tiny RDBMS(MySQL/PostgreSQL/SQLServer) tables. Parallelism or efficiency can be ignored since the target tables are all tiny. Some use cases:

• Some dimension tables in Hive are snapshots of RDBMS tables. Users want to query the difference between the snapshot in Hive and the latest data in RDBMS.
• Users want to run queries joining Hive fact tables and the latest data in RDBMS.
• Users hope their query results can be ingested into MySQL directly

Implement an "External Data Source" as a generic JDBC wrapper for RDBMS data sources could be a solution. The drawback is that "External Data Source" requires users to create tables in Impala for each RDBMS table they want to access. Users can't list tables (show tables) of a schema(database). 

There're other solutions that support RDBMS directly. For example https://www.slideshare.net/liuknag/cloudera-impala-postgre-sql-29025605