Integration test for JDBC driver in Grill server

Changes - 
1. Added JDBC test case as sub class of TestQueryService
2. Changed most validate* methods in TestQueryService from static to instance methods so that they can be overridden in JDBC test case
3. Made driver map in QueryExecutionServiceImpl protected so that it can be manipulated in tests
4. Fixed VARCHAR datatype in JDBC result set.
Added persistent result test case