BUG/MEDIUM: debug: fix possible hang when multiple threads dump at once

BUG/MEDIUM: debug: fix possible hang when multiple threads dump at once

A bug in the thread dumper was introduced by commit 00c27b50c ("MEDIUM:
debug: make the thread dumper not rely on a thread mask anymore"). If
two or more threads try to trigger a thread dump exactly at the same
time, the second one may loop indefinitely trying to set the value to 1
while the other ones will wait for it to finish dumping before leaving.
This is a consequence of a logic change using thread numbers instead of
a thread mask, as threads do not need to see all other ones there anymore.

No backport is needed, this is only for 2.7.