Restore code that sets delegate to null otherwise PoolGuardConnectionWrapper can't do what it is meant to.

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1568641 13f79535-47bb-0310-9956-ffa450edef68
Use a PreparedStatement for the validation query to improve performance. Testing suggests a ~15% improvement with a very basic validation query.

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1568643 13f79535-47bb-0310-9956-ffa450edef68
Add query timeout check to TesterPreparedStatement so validation query timeout test passes

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1568666 13f79535-47bb-0310-9956-ffa450edef68