OPENDJ-1602 (CR-5566) New pluggable storage based backend

Reduced coupling with JE.


AttributeIndex.java, JEBUtils.java:
Moved presenceKey constant to JEBUtils because it is unused in AttributeIndex.

IndexQueryFactoryImpl.java, VerifyJob.java:
Now use JEBUtils.presenceKey.

PresenceIndexer.java:
Added presenceKey and presenceKeyBytes constants.
Removed getPresenceKeyData(), replaced by presenceKey.
In replaceEntry(), delegate the call to modifyEntry() since code is the same.

TestBackendImpl.java:
Now use PresenceIndexer.presenceKey.