Fix statement interruption

1. Fix syntax for interruptStatement method to refer to correct `conn` object.

1. Fix SQL statement for interruptStatement to use `statementID` variable (not SQL column).

1. Add unit test to verify statement interruption.

Coffeescript modifications only. Javascript not built.

*NOTE:* Unfortunately, the normal testing method using `SELECT SLEEP(1000)` does not work since `SLEEP` statements are not interruptible. See connection_test.coffee for more comments.