Use After Free In Nemesis Shutdown Code

[insertinterestingnamehere]

There's a race condition and use after free bug in the Nemesis threadqueue termination code where a worker thread may access a queue node after the main thread has already freed it. Relevant lines are:

Read from worker thread:

qthreads/src/threadqueues/nemesis_threadqueues.c

Line 393 in d6ce514

node = qt_internal_NEMESIS_dequeue(&q->q);

Write (free) by main thread:

qthreads/src/threadqueues/nemesis_threadqueues.c

Line 399 in d6ce514

FREE_TQNODE(node);

This is part of #149.

[insertinterestingnamehere]

This appears to have been resolved by one of the other thread sanitizer patches. Probably #206. This can also be closed when that one's merged.