Privatise constants - there are public getters

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1564833 13f79535-47bb-0310-9956-ffa450edef68
constant does not need to be exposed to sub-classes

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1564836 13f79535-47bb-0310-9956-ffa450edef68