Use constants for Dynamo column names

- Should have done this in the first place!
Only save the session attributes if they have changed

- For new sessions, use PutItem, for existing sessions use UpdateItem
- Track attributes to compare values at load and at save
- If changed, save the attributes, otherwise only save lastAccessed
- Use lastAccessed from Dynamo column rather than saving lastAccessed
  in the serialized session itself.
- Guard more FINE logging with log.isLoggable()
- Bump version to 1.9.0