https://gitlab.com/gitlab-org/gitlab/-/merge_requests/21038 removed an
optimization that prevented a Redis DEL call from being issued if no
broadcast messages were present. Now, two DEL calls are issued for every
request, which led to a significant performance regression.

The Redis delete optimization was intended to prevent filtering stale
messages over and over again. However, the optimization only worked when
there were no other broadcast messages for the future. For example, if
there were 1 current message and 1 message scheduled for 5 days later,
the filtering would still need to be done.

To improve this optimization, we check to see whether the filtered set
is different from the set we retrieved from the cache. If there is a
difference, we know it's time to flush the cache.

This came out of an investigation in
https://gitlab.com/gitlab-com/gl-infra/production/issues/1722.