Finish adoption of C# 9 features

[AKlaus]

Many features of C# 9 have been already adopted, but there are some that need to be tested and rolled out further :

• Records (safe change as under the hood they are almost the same):
  • DTOs (request / response objects)
  • The RavenDB entities
• Init only setters rather than private setters
• Fit and finish features

See full What's new in C# 9.

[AKlaus]

Changes have been mostly done by now. I'll clean up the rest as I go. Closing the ticket