Cached Git HTTP clones

In this issue we discussed possible caching improvements for Git clone, in light of production#3013. In that incident we were reminded that the Git clone pressure exerted by gitlab-org/gitlab CI is too much for the Gitaly servers that host gitlab-org/gitlab. The only reason this normally works is that we have a custom Git clone caching scheme configured inside that CI pipeline.

The outcome of this discussion is the proposal in #688 (closed). In that proposal we describe a way we could have a cache in Gitaly that can deduplicate this sort of heavily parallelized clones.