Fix two null pointer Exception that can happen in the conflict resolution code
when
- replaying a modify operation on an entry that has already been deleted (778)
- replaying a modify DN operation to rename an entry below a parent that does not exist anymore. (775)

Fix the conflict resolution code in cases where it is necessary to generate a
conflicting DN following a modify DN operation because the new DN has already been
used by another added or renamed entry. (779)

Fix a race condition in UpdateOperationTest that can make the test fail on some platforms

Add some tests for modify DN naming conflicts

reviewed by Daniel