message: Fix issues in dxl.xsd that didn't meet XSD standard
  Fixed issues in dxl.xsd that didn't meet XSD standard
  * complexContent should only include (restriction|extension)
  * SubPlanTestExpr: add "dxl:" space prefix
diff: </xsd:complexType>
  <xsd:complexType name="LogicalCTASType">
  - <xsd:complexContent>
  <xsd:sequence>
  <xsd:element name="Columns" type="dxl:ColumnsType"/>
  <xsd:element name="CTASOptions" type="dxl:CTASOptionsType"/>
  <xsd:attribute name="DistributionColumns" type="xsd:string" use="optional"/>
  <xsd:attribute name="InsertColumns" type="xsd:string" use="required"/>
  <xsd:attribute name="VarTypeModList" type="xsd:string" use="required"/>
  - </xsd:complexContent>
  -
  </xsd:complexType>
  <xsd:complexType name="TableScanType">
  <xsd:group ref="dxl:PhysicalBitmapAccessPathType" minOccurs="1" maxOccurs="1"/>
  <xsd:group ref="dxl:PhysicalBitmapAccessPathType" minOccurs="1" maxOccurs="1"/>
  </xsd:sequence>
  - </xsd:extension>
  <xsd:attribute name="TypeId" type="xsd:string" use="optional"/>
  + </xsd:extension>
  </xsd:complexContent>
  </xsd:complexType>
  <xsd:complexType name="SubPlanType">
  <xsd:sequence>
  - <xsd:element name="TestExpr" type="SubPlanTestExpr"/>
  + <xsd:element name="TestExpr" type="dxl:SubPlanTestExpr"/>
  <xsd:element name="ParamList" type="dxl:SubPlanParamListType"/>
  <xsd:group ref="dxl:PhysicalOp"/>
  </xsd:sequence>
  