OAK-1333 - SegmentMK: Support for Blobs in external storage

Only store the external blob reference without the blob length to
reduce the chance of inconsistencies and to make migrations faster

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1578934 13f79535-47bb-0310-9956-ffa450edef68