MySQL: Free the results when QSqlQuery::finished() is called

MySQL: Free the results when QSqlQuery::finished() is called

Calling mysql_stmt_free_result() frees the results of the last
executed query while keeping the prepared statement valid. This
allows one to keep around prepared QSqlQueries without the overhead
of keeping all the results in memory.

Change-Id: I4589e90857cc4e9a6f9612799bfca967a67e2ab2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Gbp-Pq: Name mysql_free_results_when_qsqlquery_finished_is_called.patch