CRASH and ASSERT creating threads in standalone mode on Windows

My new test burst_traceopts for #3977 runs raw2trace inside itself while in DR standalone mode. When raw2trace uses a worker pool, I see a number of issues:

An assert `core\win32\os.c:2715 false` and another one `\core\win32\callback.c:8708 false` due to DR's thread hook getting confused as to whether it should take these threads over. When I relax those asserts and let the threads go native, I still see a crash. However, the crash does not reproduce when I run in the debugger. Since I want to finish #3977 I am filing this to figure this out at a future date, and I'm disabling the thread pool for this test for now.

The crash is just like this:

```
% clients/bin64/tool.drcacheoff.burst_traceopts.exe

Segmentation fault
```