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.
Change do_stop so it won't unlink pid_file if the pid changes midway.

Originally the code would kill pidfiles for standbys being promoted
to active workers in HA scenarios.
Remove un-necessary variable.
Remove check against read_pid, which could result in unkilled processes.

Also expand comment about pid_file removal.
Merge pull request #53 from atomicstack/master

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