Two changes:
1. Move bin/web to bin/web_unicorn(removed PUMA switching logic)
Introduce new shadow script bin/web. Now we have 3 scripts working
together: bin/web, bin/web_puma, bin/web_unicorn

  In bin/web, it checks ENV['USE_WEB_SERVER']:
    - if value is 'puma' or value is not set, call bin/web_puma
    - if value is 'unicorn', call bin/web_unicorn
    - report error if other values given

2. Gitlab rails allow ENV setting to override timeout
    - ENV['GITLAB_RAILS_RACK_TIMEOUT'] for service_timeout
    - ENV['GITLAB_RAILS_WAIT_TIMEOUT'] for wait_timeout

Because there will be similar observer for PumaWorkerKiller,
it makes sense to keep both on better place.

It assures that requests are aborted after 60 seconds, otherwise
an exception is raised. This exception is logged by Sentry, also
there is a Prometheus counter for measuring number of requests in each
state.