RebindTestFixture: support for dumping mementoDir

- Create a mementoDirBackup field, in case wants to be used by
  sub-classes.
- Adds dumpMementoDir(File dir)
More logging for failing BindDnsServerIntegrationTest

- Fails non-deterministically on jenkins; can’t reproduce locally.
  Therefore adding logging to dump out a copy of the persistence dir,
  as it was at the point of rebind.
- Also terminates the old management context.