QSqlResult: replace virtual hooks with virtual functions

Instead of virtual functions which would have broken binary
compatibility, virtual_hook() was used to implement the virtual
functionality. Now, since the step to Qt 5.0 allows breaking
binary compatibility, we take the opporunity to simplify the code
using real virtual functions.

SetNumericalPrecision --> setNumericalPrecisionPolicy()
NextResult --> nextResult()
DetachFromResultSet --> detachFromResultSet()
BatchOperation --> execBatch()

Task-number: QTBUG-25252
Change-Id: Idd3a870f876d8b8a7457559d5f31ec2073786a75
Reviewed-by: Bill King <bill.king@nokia.com>
Reviewed-by: Harald Fernengel <harald.fernengel@nokia.com>