Reduce the number of calls to /jcr:system/jcr:versionStorage during the upgrade

Each time a version history is copied during the upgrade, the copier tool has to traverse /jcr:system/jcr:versionStorage ancestors to get the appropriate nt:versionHistory node. It can be optimized by reading the versionStorage node once and re-using it. As a result, all methods related to copying versions should accept versionStorage rather than rootNode as their parameters.