OAK-1164: Duplicate binaries

Drop the ValueBasedBlob class, and replace the functionality with
a mechanism for unwrapping a binary ValueImpl instance. The solution
is a bit ugly, but required until or unless we separate the Value
interface more cleanly from PropertyState.

Also adjust the XML import code to use ValueFactory instead of
Jackrabbit's ValueHelper and related Value classes for creating
Value instances.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1540433 13f79535-47bb-0310-9956-ffa450edef68