- Changed the signature for registerDB to ommit the unused second
  argument.  DB, the normal caller of registerDB will work with the
  old signature.

- Loosened the input requirements to invalidate to not require a
  dictionary with unused keys.

- Added a references function to give storages a way to extract object
  references from database records that will work with storage
  adapters that might change the record format, for example through
  encryption or compression.