Reinstate timeouts for killing workers

The timeout mechanism is implemented via shared tempfile handles
between the worker and master and checking the ctime of the
tempfile from the master.  Instead of using sockets or pipes to
communicate between the workers and master, this allows the
master to avoid being overloaded with wakeups when the workers
are running at full crank (or this avoids having extra logic in
workers to throttle wakeup notifications to master).

The master still wakes up at a leisurely interval of once per
second to check, reap, or murder workers that are timed out.

[1] http://cr.yp.to/docs/selfpipe.html

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