Fixes bug where using Execution the Hive sources no longer append data.

The hack we do in the Hive parquet sources no longer works so this is a work around. The benefit this has is that we can now choose which sources are append and which one's aren't.
We're still running the execution in a "sub execution" here, even when append is false. This doesn't really address my comments from earlier. 

I am happy for you to merge, though. As I said earlier, I don't have any concrete reason why you should avoid running "sub executions" unless necesary. :+1: