Export:
- Remove leftover warning about a bug that was fixed in
  commit e76af297
- In neomigrate script, open NEO storage read-only.
- IStorageIteration is already implemented.

Import:
- Review comments.
- In neomigrate script, warn that IStorageRestoreable is not implemented.
- Do not call 'close' method on source iterator. BaseStorage does not do it and
  this is not part of ZODB API. In the case of FileStorage, resource are freed
  automatically during garbage collection.