BUG/MEDIUM: thread: Fix a deadlock if an isolated thread is marked as harmless

BUG/MEDIUM: thread: Fix a deadlock if an isolated thread is marked as harmless

If an isolated thread is marked as harmless, it will loop forever in
thread_harmless_till_end() waiting no threads are isolated anymore. It never
happens because the current thread is isolated. To fix the bug, we exclude
the current thread for the test. We now wait for all other threads to leave
the rendez-vous point.

This bug only seems to occurr if HAProxy is compiled with DEBUG_UAF, when
pool_gc() is called. pool_gc() isolates the current thread, while
pool_free_area() set the thread as harmless when munmap is called.

This patch must be backported as far as 2.0.