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

[Jun Wu]

Excerpts from Augie Fackler's message of 2017-04-21 15:41:39 -0400:
> > 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.
> 
> Should we be planning to take these for 4.2, or wait and do them in early
> 4.3 cycle?

Probably after freeze. It mainly affects CI.