OAK-806: Content migration from Jackrabbit to Oak

Copy the namespace registry directly instead of going through the JCR API.
This way we can capture the stored index values and later on avoid extra lookups.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1532056 13f79535-47bb-0310-9956-ffa450edef68