Raise the Keep-Alive idle timeout to 100 seconds (#261)

Heroku's Router 2.0 now supports Keep-Alive connections:
https://www.heroku.com/blog/tips-tricks-router-2dot0-migration/#keepalives-always-on
https://devcenter.heroku.com/articles/http-routing#legacy-router-and-router-2-0

Gunicorn supports Keep-Alive connections, and so now if an app
is using Router 2.0 connections between the router instances and
the app will be kept alive.

However, gunicorn's default `keepalive` idle timeout setting is only
5 seconds:
https://docs.gunicorn.org/en/stable/settings.html#keepalive

This is shorter than the 90 second Router 2.0 keep-alive timeout:
https://devcenter.heroku.com/articles/http-routing#keepalives

As such:
(a) This causes connections to be closed sooner than they need to
    be (as noted in the gunicorn docs, the default 5 seconds is more
    suited for cases where many clients are connecting directly to
    gunicorn, rather than gunicorn being behind a load balancer).
(b) there is a possibility of a race condition whereby gunicorn starts
    initiating the closing of an idle Keep-Alive connection just at the
    moment that the Heroku Router sends a new request to it. If this
    occurred, that new request could fail.

This race condition isn't unique to Heroku or gunicorn, but applies to any
load balancer/app server combination that supports Keep-Alive. For example,
see AWS' explanation about this in their ELB docs:
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/edit-load-balancer-attributes.html#connection-idle-timeout

Therefore, the gunicorn idle timeout has been raised from 5 seconds to
100 seconds, so it is greater than the Router's idle timeout - ensuring
that the router is always the one initiating connection closing (and the
router will know to not send new requests to a connection it's in the
middle of closing).

GUS-W-18319007.

Author: edmorley

gunicorn.conf.py

@@ -49,6 +49,13 @@
 # https://devcenter.heroku.com/articles/dyno-shutdown-behavior
 graceful_timeout = 20
 
+# The number of seconds an idle Keep-Alive connection is kept open. This should be greater than
+# the Heroku Router's Keep-Alive idle timeout of 90 seconds, to ensure that the closing of idle
+# connections is always initiated by the router and not gunicorn, to prevent a race condition
+# if the router sends a request to the app just as gunicorn is closing the connection:
+# https://devcenter.heroku.com/articles/http-routing#keepalives
+keepalive = 100
+
 # Enable logging of incoming requests to stdout.
 accesslog = "-"