Commit 8446d16 released in 1.10.4 changed how some cancelled query
errors were returned. This has caused a lib/pq application I work on
to start returning "driver: bad connection". This is because we were
cancelling a query, after looking at some of the rows. This causes a
"bad" connection to be returned to the connection pool.

To prevent this, implement the driver.Validator and
driver.SessionResetter interfaces. The database/sql/driver package
recommends implementing them:

"All Conn implementations should implement the following interfaces:
Pinger, SessionResetter, and Validator"

Add two tests for this behaviour. One of these tests passed with
1.10.3 but fails with newer versions. The other never passed, but
does after this change.