[relDB] Finish work on extended_database

There are still sone TODOs left in the extended database. Since they are mostly 7 months old, there'll need to be some investigation on which still apply. Here's a list:

Improvements to security:

• The changed_model_copy in get function should be created via deep_copy
• use the self.is_new in more places.

Improvements:

• Later in filter method there is a TODO questioning whether the update part of the "update db models and calculate exact matching"-loop is necessary

Performance:

• There is a performance TODO relatively early in the filter function, details see in code

Misc:

• It should be investigated if it is necessary to always include the id in the mapped_fields of the changed_model_copy of the get function
• in model_fails_filter method: "meta_new" should possibly be assumed to be the default if the model is not in the database

All of the above:

• Improve and cleanup the write method

Start by locating the extended_database implementation and reviewing the existing TODOs in get, filter, model_fails_filter, and write. Investigate which seven-month-old items still apply, then resolve the applicable security, performance, behavior, and cleanup questions and verify the resulting database behavior.