Fix data race in test_thread_6 (#152)

thread_thread_6_mutatee.c

- Use proper barrier instead of volatile+sleep to synchronize threads before attaching mutator
- General code cleanup and extra logging

thread_thread_6.C

- Use threadsafe unordered_map to map BPatchIDs to tids
- Use dprintf and make it threadsafe
- Remove unused stackwalking
- Refactor mutatorTest into separate functions and correctly handle errors at each step
- Remove unused variables