Bump default shutdown timeout

[mperham]

The default shutdown timeout has been 8 seconds since forever. This value was based on Heroku's 10 second process timeout -- worker and web dynos had 10 seconds to shutdown before Heroku would send SIGKILL to kill them without warning.

18 months ago, Heroku extended that timeout to 30 seconds and a year ago I started recommending that people use -t 25 to use most of that window so their jobs have as much time as possible to finish before shutdown (the other 5 seconds is used to push lingering jobs back to Redis before actually exiting).

AWS's ECS also has a 30 second timeout:

https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_StopTask.html

Now I think it is time to update the default from 8 sec to 25 sec to make this move official and allow new apps and users to take advantage of the more recent standard automatically.

Questions? Concerns? Ideas?