This commit does two things:

1. Uses Go's graceful HTTP shutdown to close the listen port and wait
for all HTTP handlers to finish. This will wait up to `shutdown_timeout`
seconds, which is 0 by default, to preserve existing behavior.

2. Kicks out all long poll queries by closing down the shutdown channel,
which is monitored by the Redis key watcher.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/325114

Changelog: added