Scanning multiple result sets using Rows (NextResultSet keeps outdated cache).

[kamilkoduo]

When reading multiple result sets from sqlx.Rows using StructScan, some information obtained using costly reflect operations is cached for performance optimisation. However, different result sets in a single sqlx.Rows object might have different structures, so cache should be invalidated and rebuild after each NextResultSet call. Now this method is not overwritten, so old cache interferes with new structures blocking struct scan after first result set.