[CF-14] improve servlet filter perf

Different filters load the xml differently (resource vs file). If they all loaded the same way, I would have passed the path to the constructor and read it inside. I actually thought of doing it this way. When we configure filters we can give the path with a prefix(similar to how spring does it) like classpath://<path> or file://<path>. This way we can postpone the decision of loading (resources vs file) and do it in this class.