The shutdown signal handlers were installed before the workers were
initialized and weren't removed before the workers were deleted. This
would lead to a debug assertion and an eventual crash when a SIGTERM
signal was received outside of the expected scope.
The proper way to do this is to install the handlers only after the system
is up and running and to disable them as soon as the shutdown process
starts.
This mostly happened with the mxs621_unreadable_cnf test as it seemed to
receive a SIGTERM during the execution of the at-exit handlers.
c9badcb09c