Race condition in TaskSequencer

The fixes #4434 and #4464 did not address the core problem. An `std::thread` may return `!joinable` in only a handful of cases: ["after default construction, move() from, detach(), or join()"](https://en.cppreference.com/w/cpp/thread/thread) (or `std::swap()` with a non-joinable thread, I believe), or if called from the same thread. Obviously, waiting in a loop with a sleep won't make the non-joinable thread joinable under these conditions: the thread objects are not default-constructed, moved from, swapped or detached by `TaskSequencer`, and the current thread will remain current thread.

TL;DR: In TaskSequencer as it is written, a non-joinable thread may not ever become joinable. Yet it does.

The problem is obviously related to unsynchronized access to a common data structure, the thread list. The list has not been updated yet, and the code is hitting a wrong thread.