fix(postgres): clean up leaky cursor coming from raw_sql

[cpcloud]

Description of changes

This PR cleans up a leaky cursor coming from a raw_sql call that never closed
the cursor it returns.

I've also added some SQL that should make it easier to debug the problem of
idle transactions in the future.

[cpcloud]

Ok, this doesn't seem to address the issue yet.

I originally ran a test that was hanging (reproducibly), and then made this change and then it stopped hanging. Back to drawing (read: testing) board!

[cpcloud]

It seems that

@contextlib.contextmanager
def begin(self):
    con = self.con
    with con.cursor() as cursor, con.transaction():
        yield cursor

...

with self.begin() as cursor:
    cursor.execute(create_stmt_sql)
    cursor.executemany(sql, data)

and

con = self.con
with con.cursor() as cursor, con.transaction():
    cursor.execute(create_stmt_sql)
    cursor.executemany(sql, data)

behave differently. The latter doesn't cause an idle transaction deadlock, while the former does.