Fix various setQueryTimeout bugs.

1. If you call setQueryTimeout(5), wait 10 seconds, and call execute(), the
cancel timer has already expired, and the statement will be allowed to run
forever. Likewise, if you call setQueryTimeout(5), wait 4 seconds, and call
execute(), the statement will be canceled after only 1 second.

2. If you call setQueryTimeout on a PreparedStatement, and execute the same
statement several times, the timeout only takes affect on the first
statement.

3. If you call setQueryTimeout on one Statement, but don't execute it, the
timer will still fire, possible on an unrelated victim Statement.

The root cause of all of these bugs was that the timer was started at the
setQueryTimeout() call, not on the execute() call.

Also, remove the finally-block from the cancellation task's run-method,
because that might erroneously cancel the timer for the next query, if a
new query is started using the same statement fast enough.