RowsAffected is incorrect

[deepthi]

Overview of the Issue

We are setting RowsAffected on a query result to the number of rows returned.
This is incorrect. According to the mysql C-API, mysql_rows_affected() returns the number of rows affected by a DML.
It is not considered relevant for queries, and gets set to -1.

Reproduction Steps

Unit test query_test.go:
https://github.com/vitessio/vitess/blob/master/go/mysql/query_test.go#L418