make node dependency optional

[jensens]

so idea is

• move current record implementation with its indexers to own file. Keep the implementation, but use an extra in setup.py so node et al are only soft dependencies.
• Create new default Record by using a PersistentDict.
• Create an indexer using the new record.
• Document the options.
  this should reduce the number of objects in ZODB massive. For small records this makes sense.

This should go in a new major release.

[rnixx]

+1