qgen: rework query execution to handle CRUD queries

Since CRUD queries for Impala/Kudu will have side-effects, the random query generator framework's query execution path needs to change to support CRUD. It needs to ensure the database under test doesn't end up so polluted that queries are no longer useful. Examples here including a DELETE that deletes all the rows; UPSERT/UPDATE that reduces variability in a column; INSERTs that make blow up the size of the database; etc.

The query execution also needs to consider CRUD validation. This tasks is about laying the groundwork for that.