TAJO-847: Supporting MariaDB-based Store, which is compatible with MySQL.

I have switched TAJO-847 pull request from branch-0.8.1 to master.
Please check this version.
As @jihoonson stated, I experienced Tajo Session removal, indicating that MariaDB JDBC disconnection, when I tried to execute a query directly after dropping table schema, as following: 

**NOTE! this log is only after applying latest TAJO-847 patch**
2014-06-17 12:48:35,466 INFO org.apache.tajo.master.GlobalEngine: =============================================
2014-06-17 12:48:35,489 INFO org.apache.tajo.catalog.CatalogServer: relation "linewalks.test" is deleted from the catalog (192.168.0.15:26005)
2014-06-17 12:48:35,504 INFO org.apache.tajo.master.GlobalEngine: relation "linewalks.test" is  purged.
2014-06-17 12:48:43,526 INFO org.apache.tajo.master.GlobalEngine: Query: select * from table20
2014-06-17 12:48:43,526 INFO org.apache.tajo.master.GlobalEngine: hive.query.mode:false
2014-06-17 12:48:43,529 INFO org.apache.tajo.catalog.store.MariaDBStore: Conn abortion when checking isValid; retrieve false to create another Conn.
2014-06-17 12:48:43,565 INFO org.apache.tajo.master.GlobalEngine: =============================================
2014-06-17 12:48:43,565 INFO org.apache.tajo.master.GlobalEngine: Optimized Query:
…
2014-06-17 12:48:43,565 INFO org.apache.tajo.master.GlobalEngine: =============================================
2014-06-17 12:48:47,272 INFO org.apache.tajo.master.session.SessionManager: Session a1121c5d-51ea-4397-b345-e062cd32350f is removed.

[Error description]
In other words, the query sequence always incurs NullPointerException in invoking Conn.isValid method when using up-to-date MariaDB JDBC Connector.
[Solution]
To avoid the drawback, I have agreed with @jihoonson to insert MariaDBStore-specific NPE handling, isConnValid method; which is a temporal solution until MariaDB JDBC resolves disconnection issue.