Improve safety of lazily initialized settings (#37849)

- Introduce an "init" hook for application settings.
- Add guards against introducing race conditions again in other ways.
- Fix the usage of hygienic scope macros with read only settings.
- Lots of misc improvements to settings API and implementation.

This init hooks better ergonomics, and protects against race conditions. 
The previous pattern of calling setters inside getters almost caused a P0. 

See https://github.com/metabase/metabase/issues/37801.