OAK-1604 - Support for signed references in Blob

- Update BlobStore interface with reference mgmt related methods
  - getReference(blobId)
  - getBlobId(reference)
- DataStoreBlobStore delegate the calls to DataStore specific impl
- Default impl return null

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1581718 13f79535-47bb-0310-9956-ffa450edef68
OAK-1604 - Support for signed references in Blob (WIP)

Update SegmentNodeStore to properly handle references
-- Distinguish between Segment notion of 'reference' which refers to any
   externally stored blob vs Blob#reference which refers to a 'secure reference'
-- Use blobId where raw blobid is required like in ReferenceCollector

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1581719 13f79535-47bb-0310-9956-ffa450edef68