OPENDJ-1090 ECL changenumbers get reset after a purge and server restart


Never trim the newest (last) record in the ChangeNumberIndexDB from production code.
This last record contains the last published change number, which is the one that must be read on startup and used to compute the following monotonically increasing change numbers.


JEChangeNumberIndexDB.java:
Improved and fixed javadocs.
In ctor, factorized code doing twice the same thing. 
In addRecord(), updated the newestChangeNumber after a newest record is committed.
In clear(DN, AtomicBoolean), updated the oldestChangeNumber + do not trim the newest record from the DB.

JEChangeNumberIndexDBTest.java:
Updated testClear() to assert the newest record is never trimmed by production code.