[PATCH 1 of 2 STABLE] worker: be silent if killed by the main process

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-04-22 12:18:52 +0900:
> > The new plan is to use SIGUSR2 to "kill" workers, SIGUSR2 handler is to do
> > sys.exit. So "killed!" is removed and traceback is kept.
> 
> What's the key difference from just making SIGTERM handler silent?

No "killed!" is printed. And users killing them manually will still get
"killed!".