Establish keys subset to shard out of SharedState

In https://gitlab.com/gitlab-com/gl-infra/capacity-planning/-/issues/223 we discussed the precarious state of the SharedState Redis shard CPU saturation, and decided that sharding out a subset of keys from it was the most effective way to address this issue short-term. We now need to establish which keys should we make a candidate for sharding out.

Status 2022-10-01

Waiting to see the impact that the pubsub optimization will have #1867 (closed)