SLING-3148 Merge back from whiteboard/fmeschbe/featureflags/resourceresolver

- Add support for checking feature flags while accessing a resource
- Setup to have dynamic Feature Flag API dependency

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1562007 13f79535-47bb-0310-9956-ffa450edef68