SLING-3350 Simplify API and implementation:

- Remove ClientContext interface (clients directly use Features service)
- Add feature flag caching in the ExecutionContext
- Improve API doc
- Merge service implementations into FeatureManager
- Remove ResourceDecorator (see issue for reasons)
- Only one filter registration (extract ResourceResolver from request attr)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1562712 13f79535-47bb-0310-9956-ffa450edef68