Projects::MergeRequestsController#index makes 100 redis calls

According to this Kibana visualization, the Projecst::MergeRequestsController#index controller action is above our 500ms P90 target as of this writing (2020-07-19).

This controller action is responsible for listing merge requests (e.g. https://gitlab.com/gitlab-org/gitlab/-/merge_requests).

Loading https://gitlab.com/gitlab-org/gitlab/-/merge_requests with performance bar, it's noticeable that it makes 100 redis requests but the time spent is not that much (43ms) but reducing that number of redis calls could potentially reduce load on redis.