OPENDJ-1116 Introduce abstraction for the changelog DB


Tightening up the interfaces after review from Matthew Swift

CNIndexRecord.java:
Now final, removed a useless call to super().

ChangeNumberIndexDB.java, JEChangeNumberIndexDB.java:
Removed methods from the interface that ought not to be exposed to clients but remain internal to the implementation.
Moved javadocs to the implementation.

JEChangelogDB.java:
Reference JEChangeNumberIndexDB instead of ChangeNumberIndexDB due to the change above.