This will make io.Copy from a streamcache entry into a network socket more efficient. It will not necessarily be faster than a regular io.Copy, but the Gitaly server will spend fewer CPU cycles copying the data.
This is part of gitlab-com/gl-infra/scalability#1217 (closed) and split of from !3758 (merged). Without !3758 (merged), the code in this MR will never be called.