#127 get_or_create is refactored and also with new name

[opsdep]

@akorosov
Since you mentioned the refactoring is needed in the end of the issue #127 (comment) in #127 , I have refactored it.

[akorosov]

Refactoring is poor:

• The name update_or_ingest is ambiguous. What does this function do?
• The names update_options_values_by_default_char_fields and update_options_values_by_default_foreign_keys are too long.
• Behaviour of these functions depend on input parameter existing_ds. It should not be like that - function should not be ambiguous.
• Only two functions are created. These two functions are large. There are no unit tests for them.
• The remaining update_or_ingest is also too large.

Refactoring has to be redone from scratch and discussed before implementation.