OAK-1514: Support for Sling JcrResourceListener
- Add EventHandler.enter and EventHandler.leave
- Slight tighten the EventHandler contract
- Add injectors for Observers to Oak and Jcr

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1574896 13f79535-47bb-0310-9956-ffa450edef68