qgen: rename instances of query to statement

The random query generator uses the name "query" incorrectly.

• in class definitions
• class attributes, local variables, formal parameters
• module names

Really, it generates SELECT statements, and soon the whole beast will be generating DML statements. The name query is misleading. But to change this in every single place along with a patch set that introduces new functionality results in a very large diff, hard to read. It would be easier to read and test such a change in isolation.