Heavy simplifications for drivers

src/qir/ast/QIRDBNode.java

src/qir/driver/DBDriver.java

src/qir/driver/IQIRVisitor.java

src/qir/driver/QIRCompileVisitor.java

src/qir/driver/QIRFreeVarsVisitor.java

src/qir/driver/QIRGreedyReduceVisitor.java

src/qir/driver/hbase/HBaseDriver.java

src/qir/driver/hbase/QIRHBaseQueryVisitor.java

src/qir/driver/sql/ConnectionData.java

src/qir/driver/sql/OracleDriver.java

src/qir/driver/sql/PostgreSQLDriver.java

src/qir/driver/sql/QIRSQLStringVisitor.java

src/qir/driver/sql/SQLDriver.java

src/qir/driver/sql/SQLStringDriver.java