OAK-806: Content migration from Jackrabbit to Oak

Add an upgrade mode to oak-run. For now it only does Jackrabbit 2.x to TarMK upgrades.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1532495 13f79535-47bb-0310-9956-ffa450edef68