Abort the kill loop in do_stop if $self->pid changes midway.

If we're running in a high-availability situation, this prevents
do_stop from terminating a standby worker, in addition to the currently
exiting active worker, from getting taken out by the kill loop,
after it has promoted itself to the active worker as a result of
running `$main_worker stop`. We do this by caching the PID we first
encounter, and aborting the kill loop immediately if we see it change
while we're inside the kill loop.