Return null if private hash does not exist in VersionedGraph:getPrivateHash(Vertex v).

- If no private hash exist return null.
- Added javadocs to the method.
Keep VersionedVertex forVersion to latest when vertex is added/modified.

When a VersionedVertex is added or modified, forVersion is kept synchronized
with the latest graph version in order to retrieve the latest vertex data
when vertex.getProperty* methods are invoked.