optimize request timeout settings for heroku

* use Rack::Timeout, set to 10s
* set unicorn timeout to 15s

per advice from Heroku support, this should both reduce the incidence/impact of dyno pileups, and should provide more visibility into the root causes of slowdowns.

in a preliminary loadtest (at 2x-3x peak prod load) there were still a few pileups, but we did indeed gain intelligible and useful error reporting in New Relic, so this should definitely help us narrow down our stability investigations.