OPENDJ-2480 Upgrade fails while migrating JE backend

When opening the JE environment to remove the old indexes,
JE tries to deserialize the comparators embedded in the indexes.
In this particular case, JE cannot find the class org.opends.server.backends.jeb.AttributeIndex$KeyComparator which is used for attribute indexes.
This class has been removed from OpenDJ code with OPENDJ-2337 (Remove old JE local-db backend code).
Note that the same problem can also happen with VLV indexes org.opends.server.backends.jeb.VLVKeyComparator.

The fix is the same as OPENDJ-1637: Ensure that classes exist in the classpath that will allow deserialization of the comparators.

AttributeIndex.java, VLVKeyComparator.java: ADDED to opendj-legacy project