OPENDJ-1483: On Windows, problem with changelog's file rotation when running 
  replication topology with file-based changelog
OPENDJ-1487: File based changelog : cursors opened when clearing the log

[Note: real merge of all changelog.file package content to be done in one shot in
 a future commit]

Log.java :
* Ensure cursors opened on log are temporarily disabled before rotating
 the head (to avoid renaming the file while reader are opened on it)
* Allow cursors to be opened on log when performing a clear on the log, 
 turning them into empty cursors.