OAK-1392: SegmentBlob.equals() optimization

Check blob lengths in AbstractBlob.equal() before comparing the actual bytes.
Based on patch by Alex Parvulescu.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1567592 13f79535-47bb-0310-9956-ffa450edef68