Reported by Richard Stanway,
http://mailman.nginx.org/pipermail/nginx/2017-March/053296.html.

This has somehow escaped from fbdaad9b0e7b.

The directive configures a timeout to be used when gracefully shutting down
worker processes.  When the timer expires, nginx will try to close all
the connections currently open to facilitate shutdown.

There is no need to cancel timers early if there are other timers blocking
shutdown anyway.  Preserving such timers allows nginx to continue some
periodic work till the shutdown is actually possible.

With the new approach, timers with ev->cancelable are simply ignored when
checking if there are any timers left during shutdown.