Checks for and drops delayable signals that arrive while a thread is exiting, which is a fragile time with data structures in inconsistent states.
Tested on the threadsig app with 50 threads, where the assert was seen every time before on a local machine and is now gone. I do not think we can reliably happen to hit this with a regression test on the CI machines where core counts are lower and load is higher, and we can't easily force it with application synchronization since it relies on signals arriving while in DR code, so I'm living with just the manual test.
Fixes #4438 (closed)