TestCheckpoint fails on Windows

The new unit test that tests periodic checkpointing fails on Windows. The problem is that the method FSEditLog.purgeEditLog() uses renameTo() to atomically move edits.new to edits

File.renameTo() fails on Windows is not atomic: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4017593

I propose that we change this code to first try the renameTo(). If it fails, then it deletes edits and then renames edits.new to edits.