OAK-943 - Provide more details with CommitFailedException

The conflict details are logged so as to avoid exposing properties to end user which might not have access to it.
Exception would refer only to the path under which conflict is detected

- It unifies the conflict names between ConflictAnnotatingRebaseDiff and AnnotatingConflictHandler
- Replaced the usage of conflict names via string constant with Enum.
- Passes the tree as part of ConflictValidator to allow it to access conflict related node

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1511233 13f79535-47bb-0310-9956-ffa450edef68