MINOR: threads: Be sure to remove threads from all_threads_mask on exit

MINOR: threads: Be sure to remove threads from all_threads_mask on exit

When HAProxy is started with several threads, Each running thread holds a bit in
the bitfiled all_threads_mask. This bitfield is used here and there to check
which threads are registered to take part in a specific processing. So when a
thread exits, it seems normal to remove it from all_threads_mask.

No direct impact could be identified with this right now but it would
be better to backport it to 1.8 as a preventive measure to avoid complex
situations like the one in previous bug.