Check StatementPrefetchIterator::fetchCol after strenghtening of ::fetchField

Fixed and added test coverage.

Let's add deprecation for StatementPrefetchIterator::fetchColumn. On it.

Added deprecation and draft CR.

I have just a single remark on the MR.
I have updated the IS and the CR.

All code changes look good to me.
My remark has been addressed.
The IS and the CR are in order.
For me it is RTBC.