Invalidate cache on Rows.NextResultSet call

[kamilkoduo]

When reading multiple result sets from sqlx.Rows using StructScan, some information obtained usCCCing 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 after first result set scan.

Relates #857

[cornelk]

Thanks for the fix! There is another PR open for this issue: https://github.com/jmoiron/sqlx/pull/747 which includes a unit test.

@jmoiron any chance to get this or the other fix merged?

[batazor]

@jmoiron Yeah, that's a real problem, it would be cool if you could merge one of these solutions

[tazimmerman]

@jmoiron Are you open to additional maintainers? There are a handful of really useful PRs that look straightforward and I'd be happy to help get them merged. This is a fantastic project and given its widespread use, keeping future changes in the original repo would be ideal.

[dlsniper]

Hello, @ardan-bkennedy, and I recently stepped in to help maintain this project. We are currently evaluating all the opened PRs. We'll take a look at this shortly and see if this, #747 or another, will be a good solution for this problem.