Checkpoint commit for OPENDJ-1471 File based changelog : improve cursor behavior

First step : improve cursor behavior for file-based implementation only

* Log.java, LogFile.java, FileReplicaDBCursor.java : change cursors to behave 
   like java.sql.ResultSet, ie cursor is positionned before the first record
 
* FileReplicaDBCursor.java : fix behavior for case when record with
 start key is not available when cursor is created 

* FileChangeNumberIndexDBCursor.java : update to adapt to new behavior
  of underlying cursor, but still behave differently than  java.sql.ResultSet
  (to be changed in the next step)

* LogTest.java, LogFileTest.java : adapt tests to new behavior

* FileReplicaDBTest.java : update test related to exhausted cursor 
 for better coverage of edge cases