Race condition during AppLogic::ReloadSettings and all reads of _settings

[lhecker]

Windows Terminal version

All of them

Windows build number

No response

Other Software

No response

Steps to reproduce

Reduce startup cost and make Windows Terminal launch faster.

Expected Behavior

No response

Actual Behavior

Windows Terminal will randomly crash because now that it starts a lot faster (relatively speaking) the main- and window-thread(s) will call ReloadSettings() concurrently.

Additionally, we swap out the _settings member on the main thread while window threads may currently be reading from it. This is problematic because after optimizations the pointer to _settings may be cached in a register or on the stack and so we may end up reading from a settings instance that has since been deleted.

Sounds like a rare occurrence, but with just the right timing (which I now have), this crashes the app 99% of the time during startup. I suspect it also happens on main, but simply less often.