OAK-1126 Same node and property name support

- added org.apache.jackrabbit.oak.api.Descriptors and expose them already on the ContentRepository
- simply set the SNNP descriptor based on the NodeStore implementation
- adding simple SNNP tests

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1541782 13f79535-47bb-0310-9956-ffa450edef68