OAK-806: Content migration from Jackrabbit to Oak

Use the namespace mappings directly instead of going through
NamespaceRegistry. This is one fewer strict jackrabbit-core
dependency in the upgrade code.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1573023 13f79535-47bb-0310-9956-ffa450edef68