Enable gunicorn's reuse_port in production (#248)

This causes gunicorn to set `SO_REUSEPORT` on the listening
socket, which helps ensure better request distribution between
gunicorn workers and thus better performance.

We don't enable this in development, since it makes it harder to
notice when duplicate gunicorn processes have accidentally been
launched (eg in different terminals), since the "address already in use"
error no longer occurs.

See:
https://docs.gunicorn.org/en/stable/settings.html#reuse-port
benoitc/gunicorn#2938
https://lwn.net/Articles/542629/

GUS-W-17614363.

Author: edmorley

gunicorn.conf.py

@@ -62,3 +62,10 @@
 if os.environ.get("ENVIRONMENT") == "development":
     # Automatically restart gunicorn when the app source changes in development.
     reload = True
+else:
+    # Use `SO_REUSEPORT` on the listening socket, which allows for more even request
+    # distribution between workers. See: https://lwn.net/Articles/542629/
+    # We don't enable this in development, since it makes it harder to notice when
+    # duplicate gunicorn processes have accidentally been launched (eg in different
+    # terminals), since the "address already in use" error no longer occurs.
+    reuse_port = True