OPENDJ-1116 Introduce abstraction for the changelog DB

Fixed nightly test org.opends.server.replication.server.ExternalChangeLogTest#ECLReplicationServerFullTest14 broken by r9505.

r9505 allowed code to throw ChangelogException and removed several useless catch(), in particular, a catch was removed from DraftCNDbHandler's methods getPreviousCookie(), getCSN(), getBaseDN() which were predecessors of getRecord().
Older code was throwing an exception, then catching it and returning null in some places while in other places, the exception was allowed to propagate.
The final change removed the unnecessary exception throwing and swallowing where it was useless. The exception is now thrown again where it is useful.


DraftCNDB.java:
In DraftCNDBCursor(long) ctor, removed throwing the exception and return an empty cursor instead.

DraftCNDbIterator.java:
In DraftCNDbIterator() ctor, throw an exception when the cursor is empty.